Expand guide_colourbar() tick options

[wpetry]

Enable 'tick' argument to take additional arguments to control colour and size as in element_line(). For example, the default tick color is incompatible with colour bars containing light greys:

data("faithfuld")
ggplot(faithfuld, aes(waiting, eruptions)) +
       geom_raster(aes(fill = density)) +
       scale_fill_gradient(low = "white", high = "grey70")

[hadley]

Yes, this really should be a theme setting.

[thomasp85]

Do you want a full addition of all colourbar-specific theme settings. Current theme has been written with guide_legend in mind...

[wpetry]

I agree with @thomasp85 that enabling these settings in guide_colourbar() would make a bit more sense than putting them in theme() because ticks aren't common in other legend types.

Currently, the only setting to control ticks in the colour bar reside in guide_colourbar(). Perhaps the most consistent/least invasive solution would be to expand the ticks argument beyond a boolean? Something like guide_colourbar(ticks = element_line(...))? Here element_line() could be populated with arguments to match the current behaviour of the colour bar (i.e., size = {something thin}, colour = "white"). Users who desire to suppress ticks entirely would simply use guide_colourbar(ticks = element_blank()).

This gets a bit confusing because the colour options for the legend key border are in theme(legend.key = element_rect(...)). For example, a user wanting a black border around their colour bar and black ticks in the colour bar would control these in two different functions.

[hadley]

Fixed in #2541

[lock]

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