Feature request: default plot argument for `layer_*()` functions

[teunbrand]

I'm estimating that about 4 out of 5 times that layer_data(), layer_grob() or layer_scales() is used, my intention is to get that information from the plot currently on display.

I think it is a good default to have plot = last_plot() be the default value for these functions, for the following reasons:

• The current default, i.e. no value, is useless in 5 out of 5 times.
• I can think of no alternative default value that might be useful.
• It may save folks 11 keystrokes.
• For people that don't know about last_plot(), as I did for a long time, using last_plot() is more convenient than using

p <- ggplot(...) + ...
layer_data(p)

[thomasp85]

I've never really used these functions outside of unit tests and I do consider them semi-private...

but, I see no harm in introducing a default

[teunbrand]

I use it mostly to debug the data when a plot doesn't turn out as I'd expected, so often I'd need the current plot for that. For this use-case, having the last plot as a default is convenient.

There are a few (less legitimate?) use cases outside debugging as well, when you'd like to compute something on whatever a stat has computed. An example of this is: you draw a density plot, and use the density layer's data to identify the group-wise mode to plot a line at that point as an annotation.

[yutannihilation]

I too use them for debugging purposes, so I think I will also find this useful. Anyway, as it seems we all agree adding this feature should make no harm, I'm going to approve #5187.