gradient scale is converted to discrete scale when changing scale order

[BartJanvanRossum]

When changing the legend of a legend item with a gradient scale, the gradient scale is converted to a discrete scale. The example below shows this behavior for _colour_gradient2, but it also happens for other gradient scales.

If a plot has multiple scales, i.e. when reordering becomes relevant, just specifying the order of non-gradient scales, leaves the scale intact, but specifying the order of the gradient scale causes the behavior in the example below.

library(ggplot2)

set.seed(1)
df <- data.frame(
  x = runif(100),
  y = runif(100),
  z1 = rnorm(100)
)

ggplot(df, aes(x, y)) +
  geom_point(aes(colour = z1)) +
  scale_colour_gradient2()

ggplot(df, aes(x, y)) +
  geom_point(aes(colour = z1)) +
  scale_colour_gradient2() +
  guides(colour = guide_legend(order = 1))

^{Created on 2023-07-21 with reprex v2.0.2}

[smouksassi]

use guide_colorbar

library(ggplot2)
set.seed(1)
df <- data.frame(
  x = runif(100),
  y = runif(100),
  z1 = rnorm(100)
  )
ggplot(df, aes(x, y)) +
  geom_point(aes(colour = z1,shape="A")) +
  scale_colour_gradient2() +
  guides(colour = guide_colorbar(order = 1))

^{Created on 2023-07-22 with reprex v2.0.2}

[teunbrand]

Indeed, as @smouksassi indicates. This is not really a bug as the scale remains continuous but displays a discrete legend. I can see how this is surprising if you're unfamiliar with different guide types though.