An alpha transparency value can be specified to a color, in order to get better color for background.
vector of any of the three kinds of R color specifications, i.e., either a color name (as listed by colors()), a hexadecimal string of the form "#rrggbb" or "#rrggbbaa" (see rgb), or a positive integer i meaning palette()[i].
a value in [0, 1]
a vector of colors in hexadecimal string of the form "#rrggbbaa".
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