Combining Colors and Patterns

Palettes that combine color and hatching

In the Albums de Statistique Graphique, a class on a map or a bar in a chart was rarely distinguished by color alone. Cheysson’s palettes combine colors with hatching: solid fills, stripes at different angles and spacings, and crosshatching, sometimes in two colors.

Each element of a palette in cheysson_patterns is therefore a small bundle of properties. Here are the six elements of palette 1886_28:

pats <- cheysson_pattern("1886_28")
data.frame(
  pattern       = cheysson_pattern_params(pats, "pattern_type"),
  fill          = cheysson_pattern_params(pats, "fill"),
  pattern_fill  = cheysson_pattern_params(pats, "pattern_fill"),
  pattern_fill2 = cheysson_pattern_params(pats, "pattern_fill2"),
  pattern_angle = cheysson_pattern_params(pats, "pattern_angle")
)
#>   pattern        fill pattern_fill pattern_fill2 pattern_angle
#> 1  stripe transparent      #d61d21       #d61d21            90
#> 2  stripe transparent      #53679b       #53679b             0
#> 3  stripe transparent      #060304       #060304             0
#> 4    none     #53679b      #53679b       #53679b            45
#> 5    none     #d61d21      #d61d21       #d61d21            45
#> 6    none     #060304      #060304       #060304            45

• pattern: the hatch type, "none" for a solid fill
• fill: the paper or fill color behind the hatching ("transparent" for hatching on bare paper)
• pattern_fill: the color of the hatch lines
• pattern_fill2: the color of a crosshatch’s second set of lines (the same as pattern_fill except in two-color crosshatches)
• pattern_angle: the angle of the lines

The long way: one mapping and one scale per property

In ggplot2 with ggpattern, each of these properties is a separate aesthetic. To apply a palette to a variable, each aesthetic needs both a mapping in aes() and a scale that supplies its values:

trade <- data.frame(
  country = c("France", "England", "Germany", "Italy"),
  exports = c(2350, 3120, 2680, 1890)
)

ggplot(trade, aes(country, exports)) +
  geom_col_pattern(
    aes(fill = country, pattern = country, pattern_fill = country,
        pattern_fill2 = country, pattern_angle = country),
    pattern_colour = NA, pattern_density = 0.3, pattern_spacing = 0.025,
    colour = "black"
  ) +
  scale_fill_cheysson_pattern("1886_28") +
  scale_pattern_type_cheysson("1886_28") +
  scale_pattern_fill_cheysson("1886_28") +
  scale_pattern_fill2_cheysson("1886_28") +
  scale_pattern_angle_cheysson("1886_28") +
  labs(title = "Exports by Nation, 1885", x = NULL, y = "Thousands of francs") +
  theme_cheysson() +
  theme(legend.position = "none")

The short way: swatches and scale_cheysson()

ggCheysson registers each element of each palette as a ggpattern pattern of its own, a swatch, named "<palette>-<element>" (e.g. "1886_28-1"). A swatch draws all of the element’s properties together: its fill or paper color, hatch type, line colors and angle, as well as its own line spacing and density. So only one aesthetic needs to be mapped, pattern, and scale_cheysson("1886_28") is a single scale that gives each level of country one swatch. This draws essentially the same plot as above (the swatches’ line spacings differ slightly, following Cheysson’s; see Line spacing and density):

ggplot(trade, aes(country, exports, pattern = country)) +
  geom_col_pattern(colour = "black") +
  scale_cheysson("1886_28") +
  labs(title = "Exports by Nation, 1885", x = NULL, y = "Thousands of francs") +
  theme_cheysson() +
  theme(legend.position = "none")

The other pattern aesthetics, such as pattern_fill or pattern_angle, aren’t needed, and a swatch ignores them if they are mapped or set. The exceptions are pattern_spacing and pattern_density, which change how close and how thick the lines are; see Line spacing and density below.

Choosing a palette: the swatch atlas

show_swatches() draws every swatch, one row per palette and one column per element, with the palettes grouped by type. A swatch’s name is its row and column: row 1886_28, column 3 is "1886_28-3". scale_cheysson("1886_28") uses that row from left to right, and when the data have fewer levels than the palette has elements, it picks elements spread across the row (see Ordered data).

show_swatches()

The atlas shows how Cheysson built each kind of palette. In sequential palettes such as 1888_27 and 1900_28, the steps are hatchings of increasing density, ending in a solid color. In grouped palettes such as 1886_11 and 1886_24, each color comes as stripes and crosshatching, or solid and hatched. Category palettes are mostly solid colors. To see only some palettes, give their names or a type, e.g. show_swatches("diverging").

Legends

There is one scale, so there is one legend, and its keys show the full swatches. Arguments to scale_cheysson() such as name and labels go to that scale. Here a sequential palette, 1881_12, whose three steps are hatchings of increasing density in a single color, distinguishes three kinds of transport:

infrastructure <- data.frame(
  region = rep(c("North", "South", "East", "West"), each = 3),
  type   = factor(rep(c("Road", "Canal", "Rail"), 4), levels = c("Road", "Canal", "Rail")),
  length = c(250, 300, 450,  250, 400, 350,  300, 200, 500,  250, 350, 400)
)

ggplot(infrastructure, aes(region, length, pattern = type)) +
  geom_col_pattern(position = "dodge", colour = "black") +
  scale_cheysson("1881_12", name = "Network") +
  labs(title = "Transportation Networks by Region", x = NULL, y = "Hundreds of km") +
  theme_cheysson()

Ordered data: sequential and diverging palettes

Sequential palettes are stored from low to high (light to dark), and diverging palettes from one extreme through the middle to the other, so reverse = TRUE flips every palette the same way. When the data have fewer levels than the palette has elements, the scales pick elements spread over the whole palette, keeping both ends.

Palette 1883_31 is diverging: two hues, each used solid and hatched. As in Cheysson’s maps, the solid fills mark the extremes and the hatched versions the milder classes:

opinion <- data.frame(
  response = factor(c("Strongly against", "Against", "For", "Strongly for"),
                    levels = c("Strongly against", "Against", "For", "Strongly for")),
  percent = c(18, 27, 34, 21)
)

ggplot(opinion, aes(response, percent, pattern = response)) +
  geom_col_pattern(colour = "black") +
  scale_cheysson("1883_31") +
  labs(title = "A Diverging Palette", x = NULL, y = "Percent") +
  theme_cheysson() +
  theme(legend.position = "none")

Missing values

Missing values get the pattern na.value, by default "none": no swatch, so the bar shows the geom’s own fill. Since every swatch covers the geom’s fill, setting fill in the geom sets the color for missing values only:

trade_na <- rbind(trade, data.frame(country = NA, exports = 1500))

ggplot(trade_na, aes(country, exports, pattern = country)) +
  geom_col_pattern(fill = "grey90", colour = "black") +
  scale_cheysson("1886_28") +
  labs(title = "Exports by Nation, 1885", x = NULL, y = "Thousands of francs") +
  theme_cheysson() +
  theme(legend.position = "none")

Line spacing and density

The swatches of a palette keep the relative line spacings Cheysson used, measured on his plates: in a sequential palette such as 1888_27 the hatching gets denser at each step (see the literacy map in vignette("guerry-maps", package = "ggCheysson")). A palette’s typical swatch has a spacing of 0.025, and all draw their lines with a density of 0.3, which suit plots of about this size.

pattern_spacing in the geom sets that typical spacing, and the other swatches keep their proportions to it; pattern_density sets the density. What they mean depends on the version of ggpattern:

• ggpattern 1.4.0 or later: they replace the defaults, so pattern_spacing = 0.05 gives lines twice as far apart.
• Earlier versions: these always supply their own values (0.05 and 0.2), so the swatches treat a setting as relative to those: pattern_spacing = 0.1 gives lines twice as far apart.

So code that sets them may draw differently when ggpattern is updated; set them only when needed. See ?cheysson_swatches for details, including pattern_units.

Using only some of a palette’s properties

A swatch always applies all of an element’s properties. To use only some of them, use the individual scales, scale_fill_cheysson_pattern() and the scale_pattern_*_cheysson() family, as in the long way above, and map only the aesthetics you want. Here the bars keep the palette’s fills, hatch types and line colors, but not its angles:

ggplot(trade, aes(country, exports, fill = country, pattern = country,
                  pattern_fill = country)) +
  geom_col_pattern(pattern_colour = NA, pattern_density = 0.3, pattern_spacing = 0.025,
                   colour = "black") +
  scale_fill_cheysson_pattern("1886_28") +
  scale_pattern_type_cheysson("1886_28") +
  scale_pattern_fill_cheysson("1886_28") +
  labs(title = "Without the palette's angles", x = NULL, y = "Thousands of francs") +
  theme_cheysson() +
  theme(legend.position = "none")

What is not included

• Line widths: all swatches use the same density, so a hatching’s darkness comes from its spacing alone. A few palettes, such as 1891_19, trade spacing against line width, and draw less faithfully.
• The scales are discrete. For continuous data, cut it into classes first, as Cheysson did.
• For plots without patterns (geom_col(), geom_point(), …), use scale_fill_cheysson() and scale_color_cheysson(), which use the palettes’ colors only.