Scope and limitations of extra.ggplot2

[Nowosad]

@mtennekes -- would you be able to let me know what is the scope and limitations of the extra.ggplot2 argument in the charts functions? For example, I've tried to use the dark theme, and the whole chart is not longer readable. Thus:

1. What is the scope of this argument?
2. Is it possible to specify a few ggplot2 options with it?

library(tmap)
library(spData)

tm_shape(nz) +
  tm_polygons("Median_income",
              fill.chart = tm_chart_histogram())

tm_shape(nz) +
  tm_polygons("Median_income",
              fill.chart = tm_chart_histogram(
                extra.ggplot2 = ggplot2::theme_dark()
              ))

^{Created on 2025-06-24 with reprex v2.1.1}

[mtennekes]

To understand the scope and limitations, take a look at what's going on in tmap:

	g = ggplot2::ggplot(comp$tab, ggplot2::aes(x = bin, y = freq, fill = color)) +
		ggplot2::geom_bar(width = 1, lwd = lwd_to_mm(scale),color = "#000000", stat = "identity", na.rm = TRUE) +
		ggplot2::scale_fill_manual(values = comp$pal) +
		theme_chart(plot.axis.x = comp$plot.axis.x, plot.axis.y = comp$plot.axis.y, scale = scale, text.color = o$chart.text.color, text.size = textsize)

	g = g + comp$extra.ggplot2

source file

At least, It is easy to change a ggplot2 option, like https://r-tmap.github.io/tmap/articles/basics_charts#additional-ggplot2-code
My knowledge about ggplot2 is limited to pinpoint what the exact possibilities are. E.g. not sure how a ggplot2 scale can be overruled.

Also with themes: it would be nice to apply the standard ggplot2 themes directly, but ideally this should be 'merged' somehow with theme_chart (source) to make it work as a small inset.

[Nowosad]

@mtennekes small followup question -- why the title is removed in the theme (https://github.com/r-tmap/tmap/blob/7ba534e287bfe2e50fdd02332a6e363b9e7366dd/R/theme_ps.R#L67)? If this like is commented out, the title is still not shown -- so this line seems not to be used...

[mtennekes]

I guess that a title is only shown in case it is defined, e.g. with ggtitle. So in that sense this theme option line can be removed. Originally, this theme was written for 'proportional symbols' (glyphs e.g. donuts), hence the abbreviation ps.

Would it make sense to 'merge' this theme with the common ggplot2 themes?

[Nowosad]

@mtennekes what do you mean by 'merge' this theme with the common ggplot2 themes? (I think the current charts style is good -- it keeps the "tmap" look).

The only suggestion from my side (if there is no side effects) is to remove the plot.title = ggplot2::element_blank() line. (See an example at https://tmap.geocompx.org/charts#fig-chart-ggplot2)

I also think we should wait for some user request/suggestions between changing it too much.

[mtennekes]

I can remove that line (it won't break anything).

With merge, I mean keep the good aspects from the ggplot2 themes, but adjust those elements that make the charts unreadable, like margins. The current default is good, but could be nice to apply ggplot2::theme_dark nicely.