Graphical parameters for `geom_smooth()` band

[teunbrand]

This PR aims to fix #6551.

Briefly, we allow setting the colour/linetype/linewidth of the confidence bands in geom_smooth(). A demonstration:

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

ggplot(mpg, aes(displ, hwy, fill = drv)) +
  geom_smooth(
    band.colour = "black",
    band.linetype = 3,
    band.linewidth = 0.5
  )
#> `geom_smooth()` using method = 'loess' and formula = 'y ~ x'

^{Created on 2025-09-26 with reprex v2.1.1}

[teunbrand]

That's right. Do you think missing parameters should be drawn from the central line?

[thomasp85]

That is how we do it for the others, right? NULL means don't draw, NA means inherit..?

[teunbrand]

I think the meaning of NULL and NA are swapped. The main reason I initially didn't take properties from the central line was because it should be hidden by default. Of course we might achieve that by setting transparent colours, 0 linewidth or blank linetypes.

[thomasp85]

oh, yeah you are right... Then just default the arguments to NA to ensure default is not to draw, and let NULL mean inherit

[teunbrand]

I'm now hiding it with band.linetype = "blank". Activating the band lines requires setting a non-blank linetype.

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

ggplot(mpg, aes(displ, hwy)) +
  geom_smooth(band.linetype = 1, linetype = 2)
#> `geom_smooth()` using method = 'loess' and formula = 'y ~ x'

band.linetype = NULL inherits from center line.

ggplot(mpg, aes(displ, hwy)) +
  geom_smooth(band.linetype = NULL, linetype = 2)
#> `geom_smooth()` using method = 'loess' and formula = 'y ~ x'

^{Created on 2025-12-04 with reprex v2.1.1}