When data=NULL and setting all aesthetics, values get dropped

[wch]

When data=NULL, it should inherit data, not replace it with a data frame with one row per panel.

# Should have no red points
qplot(mpg, wt, data = mtcars) + geom_point(data = data.frame(), x = 20, y = 3, colour = "red", size = 5, alpha=.1)

# Should have many red points overplotted - presently only has one red point
qplot(mpg, wt, data = mtcars) + geom_point(data = NULL, x = 20, y = 3, colour = "red", size = 5, alpha=.1)

# Should have many red points overplotted in each facet, one for each black point in that facet.
# Presently has only one red point per facet.
qplot(mpg, wt, data = mtcars) + geom_point(data = NULL, x = 20, y = 3, colour = "red", size = 5, alpha=.1) +
  facet_wrap(~cyl)

I think the problem is in compute_aesthetics() in layer.r.

Here are the tests that it should pass (should go in test-empty-data.r):

  # NULL should inherit data
  d <- pdata(ggplot(mtcars, aes(x=mpg, y=wt)) + geom_point() +
    geom_point(data=NULL, x = 20, y = 3, colour = "red", size = 5))
  expect_equal(nrow(d[[1]]), nrow(mtcars))
  expect_equal(nrow(d[[2]]), nrow(mtcars))

  d <- pdata(ggplot(mtcars, aes(x=mpg, y=wt)) + geom_point() +
    geom_point(data=NULL, x = 20, y = 3, colour = "red", size = 5) +
    facet_wrap(~ cyl))
  expect_equal(nrow(d[[1]]), nrow(mtcars))
  expect_equal(nrow(d[[2]]), nrow(mtcars))
  expect_equal(sort(d[[1]]$PANEL), sort(d[[2]]$PANEL))