For data, NULL != data.frame()

[hadley]

For example,

qplot(mpg, wt, data = mtcars) + 
  geom_point(data = data.frame(), colour = "red")

should display black points.

This can occur when subsetting and is annoying.

[wch]

The map_layout() function in panel.r assumes that if it gets an empty data frame, then the data should be inherited from plot_data. The problem is that there's no way to tell whether the empty data frame is passed in, or if it was just not specified in the layer. When the geom is not specified, this is what happens: layer$new calls fortify.NULL(), which returns data.frame(), and it's this data frame that can't be distinguished from an empty data frame passed in to the layer.

One possible solution is to make fortify.NULL() add an attribute to the data frame, and then test for that attribute in map_layout().

[wch]

The fix I've submitted works for this, although you need to set inherit.aes = FALSE:

qplot(mpg, wt, data = mtcars) + geom_point(data = data.frame(), colour = "red", inherit.aes = FALSE)