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.
Usage
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.
range
Output range of alpha values. Must lie between 0 and 1.