Better error messages for incorrect aesthetic mappings

[clauswilke]

I encountered some cryptic error messages that pop up in response to fairly normal user mistakes, where the user maps an aesthetic to a data column that doesn't exist. A better error message could help a lot here.

My code example below first shows the desired output and then two incorrect ways in which a user might try to generate it.

library(ggplot2)

# desired plot
ggplot(mtcars, aes(mpg, 1)) +
  geom_tile(aes(fill = stat(density)), stat = "density") +
  geom_point(position = "jitter")

# wrong, `fill` aesthetic mapped to stat-generated data column that
# doesn't exist in `geom_point()` layer
ggplot(mtcars, aes(mpg, 1, fill = stat(density))) +
  geom_tile(stat = "density") +
  geom_point(position = "jitter")
#> Error in is.finite(x): default method not implemented for type 'closure'

# wrong, `fill` aesthetic mapped to data column that doesn't exist
ggplot(mtcars, aes(mpg, 1, fill = density)) +
  geom_tile(stat = "density") +
  geom_point(position = "jitter")
#> Don't know how to automatically pick scale for object of type function. Defaulting to continuous.
#> Error: Column `fill` must be a 1d atomic vector or a list

^{Created on 2019-01-07 by the reprex package (v0.2.1)}

[clairemcwhite]

I'll work in this for dev-day

[lock]

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