Add inherit.blank argument to element constructors

[thomasp85]

This PR is an alternative fix to #1555, #1557, #1565, and #1567 compared to #1581.

It will propose the inclusion of an inherit.blank argument to the element_* constructors. The reason for this is that we might want to suppress drawing of a parent element and all its children, and then reenable some of the children again. This requires element calculation to sometimes ignore element_blank() and sometimes not. The default should be inherit.blank = FALSE while elements in the included themes should have inherit.blank = TRUE instead.

[hadley]

This seems like a reasonable approach to me

[thomasp85]

@hadley can you review?

Now you can do stuff like this (not a beautiful plot but an example):

p + theme_minimal() + theme(line = element_blank(), axis.line.y = element_line())

[hadley]

What if we automatically set inherit.blank if complete = TRUE in theme()?

[hadley]

Can you expand on this a little bit? I assume if it's TRUE the "blank-ness" will be inherited, otherwise it will skip the parent and look at the grand parent?

[hadley]

This comment needs updating too, right?

[Katiedaisey]

Edit: Just saw you updated your repo, but the following has not changed. Can you confirm my first example with your local branch?

I like this idea. It would prevent the recursive checking that #1581 requires, but your example doesn't seem to work for me.

ggplot(mtcars, aes(wt, mpg)) + geom_point() + theme_minimal() + theme(line = element_blank(), axis.line.y = element_line())

When I examine the plot theme, it seems line = element_blank() rewrites element_line() and so axis.line.y calls all NULLS. Redoing this with theme(axis.line = element_blank(), axis.line.y = element_line() also results in all NULLS.

When I try p + theme_minimal() + theme(axis.line = element_blank(), axis.line.y = element_line(color = "red")), I get:

Examining the plot, I see axis.line.y has color = "red" but isn't printing.

$ axis.line.y          :List of 4
  ..$ colour  : chr "red"
  ..$ size    : NULL
  ..$ linetype: NULL
  ..$ lineend : NULL
  ..- attr(*, "class")= chr [1:2] "element_line" "element"

Also, currently theme_classic() doesn't inherit correctly.

[thomasp85]

This has been fixed I think - truly with the latest version

[thomasp85]

I cannot confirm any problems and all changes has been pushed - can you try again and make sure you create the plot object from scratch

[hadley]

LGTM

[Katiedaisey]

@thomasp85 Sorry, I previewed the comment but apparently never posted it. Yes, I've found no problems with the last update!