Bring stat_ydensity behavior in line with stat_density

[andrewwbutler]

Addresses #4111

stat_ydensity now drops groups with fewer than 2 data points (previously 3) and displays a warning.

library(ggplot2)
table(mtcars$cyl, mtcars$am)
#>    
#>      0  1
#>   4  3  8
#>   6  4  3
#>   8 12  2
ggplot(mtcars, aes(x = factor(cyl), y = mpg, fill = factor(am))) + 
  geom_violin(scale = "width")

# warn when fewer than 2 in group
mtcars_sub <- mtcars[-31, ]
table(mtcars_sub$cyl, mtcars_sub$am)
#>    
#>      0  1
#>   4  3  8
#>   6  4  3
#>   8 12  1
ggplot(mtcars_sub, aes(x = factor(cyl), y = mpg, fill = factor(am))) + 
  geom_violin(scale = "width")
#> Warning: Groups with fewer than two data points have been dropped.

^{Created on 2020-07-01 by the reprex package (v0.3.0)}

[yutannihilation]

Generally looks good to me. Added some minor comments.

Could you also add a NEWS item for this?

[andrewwbutler]

Thanks! Committed the requested changes.

[yutannihilation]

Thanks!

Could you also add a NEWS item for this?

Sorry, I'm not sure what I was saying while I was reviewing the NEWS item... 🙄