Make layer() accept formula notation for data argument

[yutannihilation]

Fix #3138

data is processed by fortify() once and stored in layer. Then, if it's a function, it's applied to the plot data when the plot is built,

ggplot2/R/layer.r

Lines 191 to 192 in 475042e

	} else if (is.function(self$data)) {
	data <- self$data(plot_data)

So, all we have to do is to define a fortify() method that converts a formula to a function.

Usages

devtools::load_all("~/repo/R/ggplot2/")
#> Loading ggplot2

ggplot(iris, aes(Sepal.Length, Sepal.Width)) + 
  geom_point(data = ~ head(., 10))

^{Created on 2019-04-13 by the reprex package (v0.2.1)}

[thomasp85]

LGTM

[thomasp85]

There's a dependency on #3242... If you merge that one first and then update this to remove the rlang namespacing it's all good :-)

[yutannihilation]

Sure, thanks!

[yutannihilation]

It seems these diffs of theme.Rd come from #2934. It might be better to create a separate PR, but I let this included.

[clauswilke]

Actually, that PR also missed to generate a new vdiffr reference image. Do you have the time to create a separate PR that includes the updated .Rd file and the missing image? That would probably be the best approach.

[yutannihilation]

Oh, sure. Will do.

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