Feature request: enable expand arguments for continuous color and fill scales.

[atusy]

Unlike scales for continuous x or y (e.g., scale_x_continuous), those for color or fill (e.g., scale_color_gradient) is not supporting expand option. At least, I want expand = c(0, 0) to work so that color bars exactly represents the range of color or fill mappings.

The example below shows that range of color bar is wider than the range specified in limits argument even if expand = c(0, 0) is specified.

@yutannihilation figured out this is caused by https://github.com/tidyverse/ggplot2/blob/047b528c98a5852d058d77c5e188c9637c88c7f0/R/guide-colorbar.r#L336

x <- c(0, 1)
ggplot2::qplot(x, x, color = x) + 
  ggplot2::scale_color_gradient(breaks = x, limits = x, expand = c(0, 0)) +
  ggplot2::guides( # Just for the visibility
    color = ggplot2::guide_colorbar(
      barheight = grid::unit(1, "npc") - grid::unit(4, "line"),
      ticks.linewidth = 2
    )
  )

^{Created on 2019-08-29 by the reprex package (v0.3.0)}

[yutannihilation]

@yutannihilation figured out this is caused by

(I talked with atusy outside of GitHub) Sorry, probably my explanation was not exact; you don't need expand argument because the colour scale is not expanded. The ticks are shifted by 0.5 because these are the places they should be as the gradient by grid::rasterGrob() is interpolated between the middles of the tiles, not between the edges of the tiles. For example, you can see the gradation happens only between the center of the black and the center of the pink:

library(grid)

g <- matrix(hcl(0, 100, c(0, 100)),
            nrow=2, ncol=1)
# interpolated
grid.newpage()
grid.raster(g, x = 0.25, width = 0.5, height = 1)
grid.raster(g, x = 0.75, width = 0.5, height = 1, interpolate = FALSE)

^{Created on 2019-08-30 by the reprex package (v0.3.0)}

So, the short answer would be "you don't need expand, the colorbar shows the exact range of the values," but I agree this might be a bit confusing to see the bar runs over the ticks...

@thomasp85 @clauswilke Do you know there's some tricks to chop a rasterGrob at the very start and end of the gradient? Can we use clipGrob?

[clauswilke]

I was confused by this bug report because I'm certain the color scale is not expanded.

The trick to fix this issue is to increase the value of nbin, e.g. to 200. The default is too low. I've thought about fixing this properly but the math for axis placement gets quite involved and increasing nbin is just so much easier.

library(ggplot2)

x <- c(0, 1)

# default nbin = 20, looks like the color scale is expanded
ggplot(data = data.frame(x = x), aes(x, x, color = x)) + 
  geom_point() +
  scale_color_gradient(
    breaks = x, limits = x,
    guide = guide_colorbar(
      barheight = grid::unit(1, "npc") - grid::unit(4, "line"),
      ticks.linewidth = 2
    )
  )

# with nbin = 200, the effect disappears
ggplot(data = data.frame(x = x), aes(x, x, color = x)) + 
  geom_point() +
  scale_color_gradient(
    breaks = x, limits = x,
    guide = guide_colorbar(
      barheight = grid::unit(1, "npc") - grid::unit(4, "line"),
      ticks.linewidth = 2,
      nbin = 200
    )
  )

^{Created on 2019-08-29 by the reprex package (v0.3.0)}

[yutannihilation]

Ah, thanks! Since this is fundamentally a matter of resolution, nbin solution makes sense to me.

The default is too low.

Agreed. Do you think we can increase the default? It will break visual tests here and there, but I feel it's worth breaking.

[clauswilke]

I think that's a question for @hadley.

[hadley]

It's fine with me.

[yutannihilation]

Thanks, then I'll create a PR for this. (Yet to investigate how many nbin is the best, though...)

[yutannihilation]

BTW, I don't quite understand what the document of nbin is saying:

A numeric specifying the number of bins for drawing the colourbar. A smoother colourbar results from a larger value.

In terms of "smoothness", at least the examples on the pkgdown site look the same as the rasterGrobs are interpolated. The only difference is the positions of ticks:

Maybe we need another example with so complex colour palettes that the interpolation of rasterGrob won't work?

Or, does this mean the colourbar should not be interpolated on rasterGrob's side? But, interpolate = TRUE is specified at the very start of this part of code. I'm confused...

https://github.com/tidyverse/ggplot2/commit/0795c6f1e3f3d4d3b9afc0aaea176d3e7a24a7f0#diff-b0d07c1ea551cbd2b2e2b796960f3bc6R137

[yutannihilation]

I still don't see what the doc of nbin says, but I'm closing this issue. Let's revisit here when this detail matters...

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