Feature Request: Syntactic sugar for theme

[teunbrand]

I have two ideas for making themes slightly easier to use.

Element shortcuts

Often, you simply want to switch a theme element on or off.
For example if you want to put axis lines on and legend titles off, you can use:

ggplot(mpg, aes(displ, hwy, colour = drv)) +
  geom_point() +
  theme(
    legend.title = element_blank(),
    axis.line    = element_line()
  )

It would be neat if we could write:

ggplot(mpg, aes(displ, hwy, colour = drv)) +
  geom_point() +
  theme(
    legend.title = "blank",
    axis.line    = "line"
  )

Subtheme

Another idea for theme specification, is that we could have subtheme functions. So instead of writing this:

ggplot(mpg, aes(displ, hwy, colour = drv)) +
  geom_point() +
  theme(
    axis.line.x.bottom  = element_line(colour = "red"),
    axis.ticks.x.bottom = element_line(colour = "red"),
    axis.text.x.bottom  = element_text(colour = "red")
  )

We could write this:

ggplot(mpg, aes(displ, hwy, colour = drv)) +
  geom_point() +
  theme_axis_bottom(
    line  = element_line(colour = "red"),
    ticks = element_line(colour = "red"),
    text  = element_text(colour = "red")
  )

Which would then simply feed forward those arguments as the axis.{part}.x.bottom arguments to the theme(). One could have similar subthemes for the legend, the panel etc. It might also encourage a more ordered approach of theme specification, instead of the wild west that is currently theme declarations.