6  Where should the eye go?

Chapter 5 was about taking things out. This chapter is about what’s left, and which part of it a reader meets first.

A plot where everything has equal weight has no first, second or third, so the reader has to do the ranking themselves.

You’re spending their attention whether you decide to or not.

Overview

Duration 50 minutes

Questions

  • How do I get emphasis for free?
  • How do I choose colours that every reader can tell apart?
  • How do I make one group stand out against the others?

What you need this session

  • A session of RStudio open
  • Something to draw on, and something to draw with
  • The following packages installed
library(tidyverse)
library(naniar)
library(colorspace)
library(gghighlight)
library(ggrepel)
pedestrian_hourly <- pedestrian |>
        mutate(day_type = if_else(
                condition = str_starts(week_day, "S"),
                true = "weekend",
                false = "weekday"
                )
        ) |>
        group_by(sensor_name, day_type, hour) |>
        summarise(
                mean_count = mean(hourly_counts, na.rm = TRUE),
                .groups = "drop"
        )

sensor_totals <- pedestrian |>
        group_by(sensor_name) |>
        summarise(total = sum(hourly_counts, na.rm = TRUE))

6.1 Order is free emphasis

The idea: the order of your categories is a decision. If you don’t make it, the alphabet makes it for you.

Our four sensors have been alphabetical since Chapter 4.

sensor_totals |>
        arrange(desc(total))
# A tibble: 4 × 2
  sensor_name                      total
  <chr>                            <int>
1 Bourke Street Mall (South)    10053301
2 Flagstaff Station              7803769
3 Birrarung Marr                 3974341
4 Spencer St-Collins St (South)  2837121

Alphabetical order puts the third busiest first and the busiest second. The letter B is doing work that traffic should be doing.

ggplot(sensor_totals,
       aes(x = total,
           y = sensor_name)) +
        geom_col()
A bar chart of total foot traffic per sensor, in alphabetical order, so the bar lengths jump up and down with no pattern.
Figure 6.1 
ggplot(sensor_totals,
       aes(x = total,
           y = fct_reorder(sensor_name, total))) +
        geom_col()
The same bar chart sorted by total, so the bars form a clean descending staircase.
Figure 6.2

One function, and the plot went from a lookup table to a ranking.

The same works on facets, so the panels read in order:

pedestrian_hourly |>
        mutate(sensor_name = fct_reorder(sensor_name, -mean_count, .fun = max)) |>
        ggplot(aes(x = hour,
                   y = mean_count,
                   colour = day_type)) +
        geom_line() +
        facet_wrap(vars(sensor_name))
Four panels of hourly foot traffic, ordered from busiest sensor to quietest rather than alphabetically.
Figure 6.3

Sorting is usually a good idea. Not always:

  • sort when the reader is comparing
  • don’t sort when the reader is looking something up

A phone book sorted by height would be a nightmare.

NoteYour Turn 
library(tidyverse)
library(naniar)
library(colorspace)
library(gghighlight)
library(ggrepel)
pedestrian_hourly <- pedestrian |>
        mutate(day_type = if_else(
                condition = str_starts(week_day, "S"),
                true = "weekend",
                false = "weekday"
                )
        ) |>
        group_by(sensor_name, day_type, hour) |>
        summarise(
                mean_count = mean(hourly_counts, na.rm = TRUE),
                .groups = "drop"
        )

sensor_totals <- pedestrian |>
        group_by(sensor_name) |>
        summarise(total = sum(hourly_counts, na.rm = TRUE))

  1. Add fct_rev() around the fct_reorder(). Which direction reads better for a horizontal bar chart?

  2. Sort the facets by weekend traffic instead of overall traffic:

pedestrian_hourly |>
        filter(day_type == "___") |>
        mutate(sensor_name = fct_reorder(sensor_name, ___))

Does the story change?

  1. fct_infreq() sorts by how often a value appears. When would you want that instead of fct_reorder()?

Only open this if you’ve actually had a go.

1. fct_rev() puts the biggest at the top. For a horizontal bar chart I prefer that, because English readers start top left and the most important bar is the first thing they meet.

2. filter(day_type == "weekend") then fct_reorder(sensor_name, mean_count). The order changes: Birrarung Marr climbs, because a park is a weekend place.

3. fct_infreq() counts rows, so use it when the thing you care about is how often a category turns up. Bar charts of raw counts, mostly.

Takeaways

  • Alphabetical is not an order, it’s the absence of one
  • fct_reorder() on a variable your reader cares about is the cheapest emphasis available
  • Sort for comparison, don’t sort for lookup

6.2 Colour that carries meaning

The idea: colour is the channel most likely to fail silently. It can look fine to you and be unreadable to one reader in twelve.

Here’s our four sensors in ggplot2’s default colours.

p_sensors <- pedestrian_hourly |>
        filter(day_type == "weekday") |>
        ggplot(aes(x = hour,
                   y = mean_count,
                   colour = sensor_name)) +
        geom_line()

p_sensors
Four hourly foot traffic lines, one per sensor, in ggplot2's default red, green, blue and purple.
Figure 6.4

Note the filter(). pedestrian_hourly holds a weekday and a weekend row for every hour, so colouring by sensor alone gives each line two values per hour and it sawtooths, as in Chapter 2. One kind of day is enough to talk about colour, so keep those rows in their own object. Everything from here uses it:

pedestrian_weekday <- pedestrian_hourly |>
        filter(day_type == "weekday")

Those four colours arrived for free, so it’s worth looking at them properly. swatchplot() from colorspace draws a palette as blocks of colour, and cvd = TRUE adds a row for each kind of colour vision deficiency, plus a desaturated row for anyone reading a black and white printout.

swatchplot("ggplot2 default" = scales::hue_pal()(4), cvd = TRUE)
The four default ggplot2 colours drawn as blocks, with rows underneath simulating deuteranopia, protanopia, tritanopia and desaturation.
Figure 6.5

Read down a column to see what one colour turns into. Read across the Deuteranope row, the commonest kind, and the first two blocks are both olive and the last two are both blue. Four colours went in, two came out.

Here it is on the plot:

p_sensors +
        scale_colour_manual(values = deutan(scales::hue_pal()(4)))
The same four lines with the colours simulated for deuteranopia, where two pairs are now nearly identical.
Figure 6.6

Half the information is gone. This is roughly 8% of men of northern European descent, and about 0.5% of women.

And the palette you were about to reach for has the same problem. Dark2 is the one everyone moves to next. Put the two of them next to viridis, in one grid, and you can see which one survives:

swatchplot(
        "ggplot2 default" = scales::hue_pal()(4),
        "Dark2" = RColorBrewer::brewer.pal(4, "Dark2"),
        "viridis" = viridisLite::viridis(4),
        cvd = TRUE
)
Three blocks of swatches, one per palette, each showing the original colours and the same colours under three kinds of colour vision deficiency and desaturated.
Figure 6.7
  • ggplot2 default: two olives and two blues
  • Dark2: the first and fourth collapse to near identical greys
  • viridis: four blocks you can still tell apart, in every row

The Desaturated row is the quickest test in the grid. Viridis survives it because it varies lightness as well as hue: on a 0 to 100 lightness scale its four colours span 76 points, where the ggplot2 default spans 6 and Dark2 spans 8.

A palette that survives a black and white photocopier will survive most eyes.

Our four lines again, in viridis:

p_sensors + scale_colour_viridis_d()
The same four lines in the viridis palette, which runs dark purple through green to yellow.
Figure 6.8

Match the palette to the variable

Most bad colour is a type mismatch. There are three kinds of palette, and each one answers a different kind of question:

swatchplot(
        Qualitative = qualitative_hcl(5, palette = "Dark 3"),
        Sequential = sequential_hcl(5, palette = "Viridis"),
        Diverging = diverging_hcl(5, palette = "Blue-Red 3")
)
Three rows of colour blocks. The qualitative row changes hue only, the sequential row runs dark purple to yellow, and the diverging row runs dark blue through near white to dark red.
Figure 6.9
  • qualitative for unordered categories, like our four sensors. Hue changes, lightness stays put, so no category looks more important than another.
  • sequential for magnitude, like a count. Lightness runs one way.
  • diverging for distance from a midpoint, like above and below average. Two sequential palettes back to back, meeting at a neutral middle.

The last two want a plot that can carry a magnitude, so here is the same weekday data as tiles, one row per sensor and one column per hour:

p_tiles <- ggplot(pedestrian_weekday,
                  aes(x = hour,
                      y = sensor_name,
                      fill = mean_count)) +
        geom_tile()

Same data, three questions, three palettes. And a fourth tab for what a mismatch looks like.

p_sensors + scale_colour_discrete_qualitative(palette = "Dark 3")
The four sensor lines in a qualitative palette, where the four hues are equally strong.
Figure 6.10

Which sensor is this? Nothing is ranked, so nothing shouts.

p_tiles + scale_fill_continuous_sequential(palette = "Viridis")
A tile plot of weekday foot traffic by hour and sensor, in viridis, where the busy hours are the dark tiles.
Figure 6.11

How busy? Dark is more and light is less, so the two commuter peaks at Flagstaff Station show up without going to the legend.

pedestrian_weekday |>
        group_by(sensor_name) |>
        mutate(above_average = mean_count - mean(mean_count)) |>
        ggplot(aes(x = hour,
                   y = sensor_name,
                   fill = above_average)) +
        geom_tile() +
        scale_fill_continuous_diverging(palette = "Blue-Red 3")
The same tile plot showing distance from each sensor's own average, in blue through white to red, with Flagstaff Station's 8am tile dark red.
Figure 6.12

Busier or quieter than usual? White is that sensor’s own average, red is above it, blue is below. The 8am and 5pm commute at Flagstaff Station is the darkest thing on the plot.

p_tiles + scale_fill_gradientn(colours = rainbow(7))
The same tile plot as the sequential tab, coloured with rainbow(), where the colours have no readable order.
Figure 6.13

The same magnitude as the sequential tab, in rainbow(). Is green more than blue? You have to go back to the legend every time, because the colours have no order of their own.

A swatch tells you the colours are different. It doesn’t tell you how they behave once they’re spread over a real plot, in small pieces, sitting next to each other. demoplot() paints a palette into a fake plot so you can judge that before you go anywhere near your own data.

par(mfrow = c(1, 3), mar = c(1, 1, 3, 1))

demoplot(qualitative_hcl(5, palette = "Dark 3"), type = "bar")
title("qualitative / bar")

demoplot(sequential_hcl(5, palette = "Viridis"), type = "heatmap")
title("sequential / heatmap")

demoplot(diverging_hcl(5, palette = "Blue-Red 3"), type = "map")
title("diverging / map")
Three fake plots: a grouped bar chart in a qualitative palette, a contour heatmap in viridis, and a choropleth map in a blue to red diverging palette.
Figure 6.14

There are nine types to choose from: "map", "heatmap", "scatter", "spine", "bar", "pie", "perspective", "mosaic" and "lines". Pick whichever is closest to the plot you’re actually making.

It pairs with the simulators. Put the palette through deutan() or desaturate() first, and demoplot draws what that reader gets:

pal <- qualitative_hcl(4, palette = "Dark 3")

par(mfrow = c(1, 3), mar = c(1, 1, 3, 1))

demoplot(pal, type = "map")
title("original")

demoplot(deutan(pal), type = "map")
title("deuteranope")

demoplot(desaturate(pal), type = "map")
title("desaturated")
The same choropleth map three times: in a four colour qualitative palette, simulated for deuteranopia where two of the colours drift together, and desaturated where the whole map is one grey.
Figure 6.15

Under deuteranopia the pink and the teal both drift towards grey, and desaturated the map is one colour. That is what a qualitative palette is: hue does all the work. These four colours span 4 points of lightness, against viridis’s 76, so there’s nothing left to fall back on.

Which is the argument for the next section. If a reader might print your plot in black and white, or you have more than a handful of categories, you’re better off highlighting one series than colouring all of them.

TipRead more

More on checking a palette before you commit to it, in Quickly Assessing Colour Palettes.

prismatic::check_color_blindness() is the same idea as swatchplot(cvd = TRUE), from a different package, if you’d rather one call than two arguments.

hour is none of the three types. Midnight is next to 23:00, so a sequential palette puts the two closest hours at opposite ends of the scale.

There’s no good default for this. The three way taxonomy is useful and it isn’t the whole story.

NoteYour Turn 
library(tidyverse)
library(naniar)
library(colorspace)
library(gghighlight)
library(ggrepel)
pedestrian_hourly <- pedestrian |>
        mutate(day_type = if_else(
                condition = str_starts(week_day, "S"),
                true = "weekend",
                false = "weekday"
                )
        ) |>
        group_by(sensor_name, day_type, hour) |>
        summarise(
                mean_count = mean(hourly_counts, na.rm = TRUE),
                .groups = "drop"
        )

sensor_totals <- pedestrian |>
        group_by(sensor_name) |>
        summarise(total = sum(hourly_counts, na.rm = TRUE))
pedestrian_weekday <- pedestrian_hourly |>
        filter(day_type == "weekday")
p_tiles <- ggplot(pedestrian_weekday,
                  aes(x = hour,
                      y = sensor_name,
                      fill = mean_count)) +
        geom_tile()

  1. Put a palette you use at work through swatchplot(cvd = TRUE). Does it survive the Deuteranope row? Does it survive the Desaturated one?

  2. Compare two more brewer palettes in one grid:

swatchplot(
        "Set1" = RColorBrewer::brewer.pal(4, "___"),
        "Set2" = RColorBrewer::brewer.pal(4, "___"),
        cvd = TRUE
)

Better or worse than Dark2?

  1. Colour p_tiles by mean_count with a qualitative palette, scale_fill_discrete_qualitative(). You should get an error. What is ggplot2 telling you about the variable?

Only open this if you’ve actually had a go.

2. Set1 holds up better than Dark2, because its colours differ in lightness as well as hue: they span 19 points of lightness against Dark2’s 8. Set2 is pastel, spans 6, and collapses badly.

3. Error: Continuous value supplied to a discrete scale. A discrete scale wants a discrete variable, and mean_count is a number. This is the one type mismatch ggplot2 catches for you. It has nothing to say about rainbow().

Takeaways

  • The default palette fails under colour vision deficiency, and so does Dark2
  • Palettes that vary lightness survive, which is why viridis is the safe default
  • Look at a palette with swatchplot(cvd = TRUE) rather than guessing from hex codes
  • Qualitative for categories, sequential for magnitude, diverging for distance from a midpoint

6.3 Emphasis by contrast

The idea: four colours asks the reader to rank four things. Usually you want them to look at one.

Grey is not an absence of colour. It’s a demotion, and you do it on purpose.

ggplot(pedestrian_weekday,
       aes(x = hour,
           y = mean_count,
           group = sensor_name)) +
        geom_line(colour = "grey80") +
        geom_line(data = filter(pedestrian_weekday,
                                sensor_name == "Flagstaff Station"),
                  colour = "firebrick")
Four hourly lines, three in pale grey and one, Flagstaff Station, in dark red.
Figure 6.16

Two geom_line() calls, one of them filtered. gghighlight() from Chapter 5 does the same job in one line, and it’s worth having seen the manual version so it isn’t magic.

Then get rid of the legend by putting the label on the line.

sensor_labels <- pedestrian_hourly |>
        filter(day_type == "weekday") |>
        group_by(sensor_name) |>
        slice_max(hour, n = 1)

p_sensors +
        geom_text_repel(data = sensor_labels,
                        aes(label = sensor_name),
                        hjust = 0,
                        direction = "y",
                        nudge_x = 1) +
        theme(legend.position = "none") +
        coord_cartesian(xlim = c(0, 32))
The four sensor lines with each name printed at the right hand end of its own line, and no legend.
Figure 6.17

A legend makes the eye bounce between the key and the plot, once per series. A label sits where the thing is.

NoteYour Turn 
library(tidyverse)
library(naniar)
library(colorspace)
library(gghighlight)
library(ggrepel)
pedestrian_hourly <- pedestrian |>
        mutate(day_type = if_else(
                condition = str_starts(week_day, "S"),
                true = "weekend",
                false = "weekday"
                )
        ) |>
        group_by(sensor_name, day_type, hour) |>
        summarise(
                mean_count = mean(hourly_counts, na.rm = TRUE),
                .groups = "drop"
        )

sensor_totals <- pedestrian |>
        group_by(sensor_name) |>
        summarise(total = sum(hourly_counts, na.rm = TRUE))
pedestrian_weekday <- pedestrian_hourly |>
        filter(day_type == "weekday")
ggplot(pedestrian_weekday,
       aes(x = hour,
           y = mean_count,
           group = sensor_name)) +
        geom_line(colour = "grey80") +
        geom_line(data = filter(pedestrian_weekday,
                                sensor_name == "Flagstaff Station"),
                  colour = "firebrick")
  1. Put the two colours in the greyed plot through the simulator:
swatchplot(c("grey80", "firebrick"), cvd = TRUE)

Does the contrast survive better than four categorical hues did?

  1. Highlight a different sensor:
ggplot(pedestrian_weekday,
       aes(x = hour, y = mean_count, colour = sensor_name)) +
        geom_line() +
        gghighlight(sensor_name == "___")

gghighlight warns about a group_by() calculation falling back. The plot is fine.

  1. If the question were “where is busy on a weekend”, which sensor would you highlight? Is that the same answer you gave in Chapter 4?

Only open this if you’ve actually had a go.

1. Yes. Every row keeps a pale block and a dark one, the desaturated row included, because grey against one colour is a lightness difference and lightness is what survives. This is the strongest argument there is for highlighting one series over colouring four.

3. Birrarung Marr, because it’s the only sensor where the weekend beats the weekday. Same answer as Chapter 4, which is a good sign: the emphasis should follow the finding, not replace it.

Takeaways

  • Grey is a decision, not a default
  • Highlight one series rather than colouring four
  • A direct label beats a legend, because the reader stops travelling

6.4 To summarise

Three things to take out of this chapter.

  1. Order is free emphasis. fct_reorder() costs one function and turns a lookup table into a ranking.
  2. Colour fails silently. Look at it with swatchplot(cvd = TRUE), prefer palettes that vary lightness, and match the kind of palette to the kind of variable.
  3. Grey is a decision. Highlight the one series you’re talking about, demote the rest, and put the label on the line rather than in a legend.

And the habit: before you style anything, decide what you want the reader to see first. Then make that the loudest thing on the page and demote everything else.

Next up is making it land, which is the last mile: labels, themes, and getting a file out that looks the way it did on your screen.

Links