Alpha-transparency scales are not tremendously useful, but can be a convenient way to visually down-weight less important observations. scale_alpha is an alias for scale_alpha_continuous since that is the most common use of alpha, and it saves a bit of typing.

scale_alpha(..., range = c(0.1, 1))

scale_alpha_continuous(..., range = c(0.1, 1))

scale_alpha_discrete(...)

scale_alpha_ordinal(..., range = c(0.1, 1))

## Arguments

... Other arguments passed on to continuous_scale() or discrete_scale() as appropriate, to control name, limits, breaks, labels and so forth. Output range of alpha values. Must lie between 0 and 1.

Other colour scales: scale_colour_brewer, scale_colour_gradient, scale_colour_grey, scale_colour_hue, scale_colour_viridis_d
p <- ggplot(mpg, aes(displ, hwy)) +
pp + scale_alpha("cylinders")p + scale_alpha(range = c(0.4, 0.8))