This function transforms a standard color (e.g. "red") into an transparent RGB-color (i.e. alpha-blend<1).
Col(col, alpha = 0.2, locate = 0)
Color (numeric or character)
Degree of transparency (0,1)
Choose colour (with mouse)
A character vector with elements of 7 or 9 characters, '"\#"' followed by the red, blue, green and optionally alpha values in hexadecimal (after rescaling to '0 ... 255').
This only works for certain graphics devices (Cairo-X11 (x11 as of R>=2.7), quartz, pdf, ...).