Scale parameters

[teunbrand]

This PR follows up on #4271 and #5444, aiming to fix #4269.

Like before, it aims to provide the convenience of setting some scale parameters, without comitting to a full scale yet.
Unlike #5444, this PR is closer to Claus' approach in #4271.

This PR the scale_params() function that populates a new ScalesList$params field. When a new scale is added, it will get updated with the parameters in that field. The update mechanism required the most attention. Every <Scale> class now has a Scale$new() method that takes on most of the work that continuous_scale(), discrete_scale(), binned_scale() constructors were doing before. The Scale$update() method is a thin wrapper around this and should resolve gnarly situations (e.g. limit transformation).

In the demo below we can note:

• the y-limits are appropriately transformed by the sqrt transformation. If we were to use the transform first and then the limits, the result would be the same.
• We can set parameters for multiple scales at once, like colour/shape.

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2
p <- ggplot(mpg, aes(displ, hwy, colour = drv, shape = factor(year))) +
  geom_point()

p +
  scale_params("x", breaks = c(2, 4, 7)) +
  scale_params("y", limits = c(0, 50)) +
  scale_params("y", transform = "sqrt") +
  scale_params(c("colour", "shape"), guide = guide_legend(ncol = 2))

The update mechanism should also recognise invalid updates:

p + scale_params("colour", foobar = "i dont exist")
#> Error in `geom_point()`:
#> ! Problem while computing aesthetics.
#> ℹ Error occurred in the 1st layer.
#> Caused by error in `scale_colour_discrete()`:
#> ! Cannot update scale with the unknown argument:
#> • "foobar"

^{Created on 2024-11-22 with reprex v2.1.1}

[thomasp85]

Would be nice if this information was somehow incorporated into the class, rather than having to check an "unlinked" constructor function