scale_gradient's palette argument cannot be specified

[twest820]

Needed to set the direction on a gradient, which the docs indicate is can be done by setting palette.

That's broken, though. Looks like lack of checking for an existing palette argument somewhere along the call stack leads to a second palette getting inserted.

ggplot() +
  geom_point(aes(x = seq(0, 10), y = seq(0, 10), color = seq(0, 10))) +
  scale_color_gradient(palette = scales::pal_brewer(direction = -1))
Error in continuous_scale(aesthetics, name = name, palette = pal_seq_gradient(low,  : 
  formal argument "palette" matched by multiple actual arguments

A basic workaround looks to be call the scale directly and give those values to high and low though, depending on the gradient being used, that can compromise perceptual uniformity.

[teunbrand]

Hi there, thanks for the report! You're right that this doesn't work, but the error here is that palette is a documented parameter while it isn't (not that the argument doesn't work). The documentation side of things has fixed already in #5992 and can be seen in the development docs: https://ggplot2.tidyverse.org/dev/reference/scale_gradient.html.
As for the palettes, these are accessible if you use the continuous_scale() and discrete_scale() constructors. In the future, it'll also be possible to set default palettes via the theme.