Control single- vs multi-group palettes via col.default

[grantmcdermott]

Closes #598

@zeileis this PR implements your idea for a new col.default parameter that controls the behaviour for single- versus mulii-group displays. I've enacted it for most (but not all) themes, using the following heuristic:

• Turn col.default = "black" on for themes that, until now, had a qualitative palette starting with black. This includes the Okabe-Ito-based themes ("bw", "ipsum2", "broadsheet",etc.), plus a few others that conceptually favour a monochrome single series ("ipsum", "socviz", etc.)
  • These themes now drop black from their grouped displays, so a lone series reads as a clean black mark while grouped displays get a full colour palette.
• For "colour-forward" themes (e.g. "clean(2)", "dark", "web"), we leave col.default as NULL so they always use the first colour of their (existing) palette.
• The plain (theme-less) defaults ("default", "basic", etc.) are unchanged: single-group displays fall back to base R palette()[1] (i.e., black).

In table form:

Theme	col.default	Multi-group palette (1st colour)
default / basic / dynamic / tufte	—	base R palette() (black)
clean / clean2	—	Tableau 10 (blue)
dark	—	Set 2 (teal)
nber	—	custom (blue)
web	—	custom (blue)
bw / classic / linedraw / minimal	black	ggplot2 less black (salmon)
ipsum	black	custom (coral)
ipsum2 / broadsheet	black	Okabe-Ito less black (orange)
socviz	black	custom (orange)
ridge / ridge2	black	Zissou 1 (gradient fills)
float / void	black	"dark" / Tableau 10

[Copilot]

Pull request overview

Implements a new theme/tpar parameter (col.default) to control default colors for single-group (no by) plots versus multi-group palettes, and updates multiple plot types/themes to apply this logic consistently (Issue #598).

Changes:

• Add tpar/theme support for col.default and validate it in tpar.
• Update color/fill resolution for single-group displays (notably ridge outlines and box/violin/bar fills under active theme palettes).
• Expand snapshot tests to lock in per-theme single-group default color behavior.

Reviewed changes

Copilot reviewed 13 out of 45 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
R/type_violin.R	Adjust single-group fill default so theme palettes can drive fill via resolved border color.
R/type_ridge.R	Respect top-level col, add col.default fallback for single-group ridge outlines, and robustly resolve theme palettes to colors.
R/type_histogram.R	Removes hard-coded single-group border default (but see review comment about by/null_by handling).
R/type_boxplot.R	Adjust single-group fill default so theme palettes can drive fill via resolved border color.
R/type_barplot.R	Adjust single-group fill default so theme palettes can drive fill via resolved border color.
R/tpar.R	Document, register, validate, and initialize new col.default tpar option.
R/tinytheme.R	Introduce “no-black” palette variants and set per-theme col.default / palette tweaks to separate single vs grouped defaults.
R/by_aesthetics.R	Add col.default hook for single-group color resolution and make single-group fills track resolved border color for selected types.
NEWS.md	Document new single- vs multi-group default color behavior and theme palette adjustments.
man/type_ridge.Rd	Update docs to explain ridge outline color precedence and col.default fallback chain.
man/tpar.Rd	Document col.default.
inst/tinytest/test-type_ridge.R	Update/extend snapshot coverage for ridge gradient cases and outline color overrides.
inst/tinytest/test-tinytheme.R	Add snapshot loop for single-group plots across all built-in themes.
inst/tinytest/_tinysnapshot/tinytheme_single_web.svg	New snapshot for single-group default under web.
inst/tinytest/_tinysnapshot/tinytheme_single_void.svg	New snapshot for single-group default under void.
inst/tinytest/_tinysnapshot/tinytheme_single_ridge2.svg	New snapshot for single-group default under ridge2.
inst/tinytest/_tinysnapshot/tinytheme_single_ridge.svg	New snapshot for single-group default under ridge.
inst/tinytest/_tinysnapshot/tinytheme_single_nber.svg	New snapshot for single-group default under nber.
inst/tinytest/_tinysnapshot/tinytheme_single_minimal.svg	New snapshot for single-group default under minimal.
inst/tinytest/_tinysnapshot/tinytheme_single_ipsum2.svg	New snapshot for single-group default under ipsum2.
inst/tinytest/_tinysnapshot/tinytheme_single_ipsum.svg	New snapshot for single-group default under ipsum.
inst/tinytest/_tinysnapshot/tinytheme_single_float.svg	New snapshot for single-group default under float.
inst/tinytest/_tinysnapshot/tinytheme_single_dynamic.svg	New snapshot for single-group default under dynamic.
inst/tinytest/_tinysnapshot/tinytheme_single_default.svg	New snapshot for single-group default under default.
inst/tinytest/_tinysnapshot/tinytheme_single_dark.svg	New snapshot for single-group default under dark.
inst/tinytest/_tinysnapshot/tinytheme_single_clean2.svg	New snapshot for single-group default under clean2.
inst/tinytest/_tinysnapshot/tinytheme_single_clean.svg	New snapshot for single-group default under clean.
inst/tinytest/_tinysnapshot/tinytheme_single_classic.svg	New snapshot for single-group default under classic.
inst/tinytest/_tinysnapshot/tinytheme_single_broadsheet.svg	New snapshot for single-group default under broadsheet.
inst/tinytest/_tinysnapshot/tinytheme_single_basic.svg	New snapshot for single-group default under basic.
inst/tinytest/_tinysnapshot/tinytheme_minimal.svg	Update snapshot to reflect new grouped palette behavior under minimal.
inst/tinytest/_tinysnapshot/tinytheme_linedraw.svg	Update snapshot to reflect new grouped palette behavior under linedraw.
inst/tinytest/_tinysnapshot/tinytheme_ipsum2.svg	Update snapshot to reflect new grouped palette behavior under ipsum2.
inst/tinytest/_tinysnapshot/tinytheme_classic.svg	Update snapshot to reflect new grouped palette behavior under classic.
inst/tinytest/_tinysnapshot/tinytheme_bw.svg	Update snapshot to reflect new grouped palette behavior under bw.
inst/tinytest/_tinysnapshot/tinytheme_broadsheet.svg	Update snapshot to reflect new grouped palette behavior under broadsheet.

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[grantmcdermott]

P.S. Here are some example, using your four plot case from #598

First, the default (no) theme:

pkgload::load_all("~/Documents/Projects/tinyplot/")
#> ℹ Loading tinyplot
par(mfrow = c(2,2))
tinyplot(gear ~ 1, data = mtcars)
tinyplot(factor(gear) ~ 1, data = mtcars)
tinyplot(mpg ~ gear, data = mtcars)
tinyplot(mpg ~ factor(gear), data = mtcars)

Second, using "clean2" theme.

tinytheme("clean2")
tinyplot(gear ~ 1, data = mtcars)
tinyplot(factor(gear) ~ 1, data = mtcars)
tinyplot(mpg ~ gear, data = mtcars)
tinyplot(mpg ~ factor(gear), data = mtcars)

Third, using "ipsum" theme.

tinytheme("ipsum")
tinyplot(gear ~ 1, data = mtcars)
tinyplot(factor(gear) ~ 1, data = mtcars)
tinyplot(mpg ~ gear, data = mtcars)
tinyplot(mpg ~ factor(gear), data = mtcars)

[grantmcdermott]

@vincentarelbundock I realized that you might to take a look at this too, since it will effect behaviour for some themes.

[vincentarelbundock]

oh yeah, that seems like a good idea to me! I like the examples a lot.

[zeileis]

Thanks, Grant @grantmcdermott! Sorry for the slow follow-up...the last weeks were very busy with the preparations for our World Cup forecast and corresponding press work.

Regarding the col.default:

• Using a col.default that is not part of palette.qualitative works great.
• As does re-using the first palette color.
• But using the first color and dropping it in the multi-group case should be facilitated.
• I will have a closer look at this and try to come up with a suggestion.

Regarding plots with shaded areas such as type_histogram, type_boxplot, type_barplot.

• For shaded areas we should use a lighter version of col.default by default.
• type_histogram and type_boxplot do so albeit via alpha blending.
• type_barplot uses the col.default directly which will typically be rather "heavy".
• I think it would be good to use a unified approach here that uses a lighter (but still opaque) version of col.default for shading areas.
• The trick via alpha blending is nice as long as there are no grid lines. With grid lines, opaque colors would work better.
• I'm not sure what is the best place in the code to resolve this.

[zeileis]

I think it would be nice to support col.default specifications relative to palette.qualitative, e.g.,

col.default = -1, palette.qualitative = "Okabe-Ito"

which should mean: use the first color from the palette as the default color and take it out of the qualitative palette afterwards. Then we don't need to keep around extra copies of palettes without black.

In order to resolve this we would have to add something like the following code somewhere in the tpar computations:

if (is.character(palette.qualitative) && length(palette.qualitative) == 1L) {
  idx = match_palette_name(palette.qualitative, palette.pals())
  if (!is.na(idx) && idx >= 1L) palette.qualitative = palette.colors(palette = palette.pals()[idx])
}
if (is.character(palette.qualitative) && length(palette.qualitative) == 1L) {
  idx = match_palette_name(palette.qualitative, hcl.pals())
  if (!is.na(idx) && idx >= 1L) palette.qualitative = hcl.colors(palette = hcl.pals()[idx])
}
if (is.numeric(col.default) && length(col.default) == 1L) {
  idx = col.default
  col.default = palette.qualitative[abs(idx)]
  if (idx < 0L) palette.qualitative = palette.qualitative[idx]
}