count.overplot checks for overlying points defined as points
separated by a maximum of tol, a two element numeric vector of
the x and y tolerance. This defaults to 1/1000 of the plot range.
Usage
count.overplot(x,y,tol=NULL,...)
Arguments
x,y
Two numeric data vectors or the first two columns of a matrix
or data frame. Typically the x/y coordinates of points to be plotted.
tol
The largest distance between points that will be considered
to be overlying.