Colourbar does not have all colors of the scale. #2343
Description
For some data it is possible for the colourbar to cover a fraction of the colourspace smaller than that covered by the data. It can be confusing when the guide for a continuous scale does span the full mapped space of an aesthetic.
Actually, this bug is probably triggered every time there is a colourbar, but in most cases it is small and imperceptible. With colourmaps like viridis, the cases where it is obvious should only go up.
In the reprex below, I had to do some contriving (nbin=7) to reveal it.
library(ggplot2)
ggplot(faithful, aes(x = eruptions, y = waiting)) +
geom_point() +
stat_density_2d(aes(fill=..level..), geom='polygon') +
xlim(0.5, 6) +
ylim(40, 110) +
scale_fill_viridis_c() +
guides(fill=guide_colorbar(nbin=7))
The root of the issue is pretty does not guarantee that the breaks it computes are close enough to the limits that the difference is not perceptible when mapped to a colour.
A simple but partial solution is to increase the default guide$nbin parameter. This would give pretty enough wiggle room to work with and reduce incidences of the bug.
A better solution is recognising that the "raster" colourbar does not need to be generated using 'pretty' breaks, a seq(.limits[1], .limits[2], length=guide$nbin) can do. For this solution the current value of nbin need not change except for the craziest of colourmaps.