Feature request: `geom_hex()` should support variable hex sizes

[teunbrand]

The idea is to allow the size of the hex to vary along with data. In the plot below, it'd mean that the dark blue hexes would become very small and light blue hexes to remain the full size.

library(ggplot2)

ggplot(faithful, aes(waiting, eruptions)) +
  geom_hex(aes(scale = after_stat(ncount)))
#> Warning in geom_hex(aes(scale = after_stat(ncount))): Ignoring unknown
#> aesthetics: scale

^{Created on 2025-11-04 with reprex v2.1.1}

The main trouble I imagine is having this encoded in a legend key. The size of hexes are dependent on the position data (width/height in data units), whereas the legend keys cannot correctly scale along with this.

[teunbrand]

The idea comes close to this, but with hexagons instead of points.

library(ggplot2)

ggplot(faithful, aes(waiting, eruptions)) +
  geom_point(stat = "binhex", aes(size = after_stat(ncount))) +
  scale_size_continuous(range = c(0, 5))

^{Created on 2025-11-04 with reprex v2.1.1}

[teunbrand]

We can already so something like this with geom_tile()

library(ggplot2)

ggplot(faithful, aes(waiting, eruptions)) +
  geom_tile(
    aes(width = after_stat(width * ncount),
        height = after_stat(height * ncount)),
    stat = 'bin2d'
  )
#> `stat_bin2d()` using `bins = 30`. Pick better value `binwidth`.

^{Created on 2025-11-04 with reprex v2.1.1}