The colors may be specified by passing a matrix or data frame as argument
red, and leaving
In this case the first three columns of
red are taken to be
Semi-transparent colors (
0 < alpha < 1) are supported only on
some devices: at the time of writing on the
X11(type = "cairo") devices and
associated bitmap devices (
bitmap). They are supported by several
third-party devices such as those in packages Cairo,
cairoDevice and JavaGD. Only some of these
devices support semi-transparent backgrounds.
Most other graphics devices plot semi-transparent colors as fully
transparent, usually with a warning when first encountered.
NA values are not allowed for any of