This PR aims tofix#4567.

Briefly,guide_colourbar() andguide_coloursteps() use theGuideLegend class to render a key for missing values.
Note that continuous scales never automatically includeNA in breaks (AFAIK), which means that you'd have to add these manually.
It'd also will prevent some complaints that suddenly the colourbars showNAs by default (as they don't).

In the plot below you can see a demo of what these look like. Note that puttingNA as the first break will put it on top of the bar, whereas using it as any other break will put it on the bottom. However, maybe we should reverse this logic to follow the order of the other breaks?

devtools::load_all("~/packages/ggplot2")#> ℹ Loading ggplot2brks<- c(10,15,20,25,30)ggplot(mpg, aes(displ,hwy))+  geom_point(aes(colour=cty,fill=cty),shape=21)+# NA break first  scale_colour_viridis_c(breaks= c(NA,brks),labels= c("missing",brks),na.value="red"  )+# NA break last  scale_fill_viridis_c(breaks= c(brks,NA),option="inferno",na.value="dodgerblue"  )

^{Created on 2024-08-02 withreprex v2.1.1}

I'm not 100% happy with the code as-is; it feels a little bit messy. Most of the logic is captured inGuide$assemble_drawing() but it doesn't quite feel like the correct place. In addition, getting binned breaks to includeNA is messier than I'd like. Lastly, maybe we should also do this forguide_bins(), but I feel like it has less priority due to it being a more niche guide (and I'd rather not mess with binned breaks again).