tidyverse
`geom_ribbon`: aesthetics can not vary with a ribbon
Hi! I ran across the error in the title when mapping alpha to an aesthetic in geom_ribbon. ggplot2 is a mature package, so I suspect there are well-founded reasons this isn't supported, but on the off-chance, is this something that can be considered in the future?
In my use case, I was mapping confidence interval around an estimate to geom_ribbon, and wanted to have it "fade out" as it projected beyond the current date. Here's a toy reprex with the error:
library(tidyverse)
pi <- 3.1415926
tibble(x = seq(0, 2 * pi, length.out = 100)) %>%
mutate(estimate = sin(x),
ci_hi = estimate + 0.1,
ci_lo = estimate - 0.1,
# decrease alpha from pi -> 2*pi
alpha = if_else(x >= pi, 0.25 * (1 - (x - pi)/pi), 0.25)) %>%
ggplot(aes(x = x,
y = estimate,
ymin = ci_lo,
ymax = ci_hi)) +
geom_line() +
geom_ribbon(aes(alpha = alpha))
#> Error: Aesthetics can not vary with a ribbon
Created on 2021-12-07 by the reprex package (v2.0.1)
Yes, this has not been possible due to limitations with the R graphics devices. With the latest improvements, this may now be possible, though probably not trivial. It's a reasonable request though.
The way the new grid features have been implemented means that it is non-trivial to map aesthetics to e.g. gradients so it is something that will take quite some work to get right... but hopefully someday
A related feature has been implemented in ggdist: https://mjskay.github.io/ggdist/articles/slabinterval.html#gradient-plots-1
The relevant code is here: https://github.com/mjskay/ggdist/blob/d660a34bb58aaca44c0c12ab35b472ca222ee9fe/R/geom_slabinterval.R#L792-L818
It doesn't seem too complicated, at least for gradients in one dimension. The problem for the feature requested here is that it will require a 2d gradient (alpha value depends on x and y position), but I believe that's in principle possible also.