[R] "negative alpha" or custom gradient colors of data dots in scatterplot ?

[madr]

I know that by setting alpha to for example col = rgb(0, 0, 0, 0.1) it is
possible to see how many overlapping is in the plot. But disadvantage of it
is that single points are barely visible on the background. So I wonder if
there is possible to make setting that single points would be almost black,
but with more and more data on the same spot it would get more and more
whiteish. Or maybe it is possible to make sole data points black but
overlapped tending to some particular color of choice ?
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