Scatter Plots - R Graphics - page 109

Figure 5-12. Overplotting, with about 54,000 points
We can make the points semitransparent using alpha , as in Figure 5-13 . Here, we'll make them
90% transparent and then 99% transparent, by setting alpha=.1 and alpha=.01 :
sp + geom_point(alpha = .1 )
sp + geom_point(alpha = .01 )