How to change the alpha value of colours in R

Often I like to reduce the alpha value (level of transparency) of colours to identify patterns of over-plotting when displaying lots of data points with R. So, here is a tiny function that allows me to add an alpha value to a given vector of colours, e.g. a RColorBrewer palette, using col2rgb and rgb, which has an argument for alpha, in combination with the wonderful apply and sapply functions.

	## Add an alpha value to a colour
	add.alpha <- function(col, alpha=1){
	if(missing(col))
	stop("Please provide a vector of colours.")
	apply(sapply(col, col2rgb)/255, 2,
	function(x)
	rgb(x[1], x[2], x[3], alpha=alpha))
	}

view raw add.alpha.R hosted with ❤ by GitHub

The example below illustrates how this function can be used with colours provided in different formats, thanks to the col2rgb function.

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