Calculate Two-Dimensional Histogram
DESCRIPTION:
Returns a structure for a 2-dimensional histogram which can be given to a graphics function such as image or persp. The user may supply an explicit list of breakpoints, or a suggested number of classes.
USAGE: 
hist2d(x, y, xbreaks=<<see below>>, ybreaks=<<see below>>,
        nxbins=<<see below>>, nybins=<<see below>>, scale=F)
REQUIRED ARGUMENTS:
x,y:
vectors representing the two dimensions of the data. x can give both dimensions by being a list with x and y components, or a matrix with two columns (the first column is x, the second column is y). Missing values are not allowed.
OPTIONAL ARGUMENTS:
xbreaks:
breakpoints for the bins on the x coordinate of the histogram.
ybreaks:
breakpoints for the bins on the y coordinate of the histogram.
nxbins:
approximate number of bins into which to place the x data, nxbin is ignored if xbreaks is supplied.
nybins:
approximate number of bins into which to place the y data, nybin is ignored if ybreaks is supplied.
scale:
if TRUE, z is scaled so that the volume under the histogram is 1. If FALSE, z contains the actual counts.
VALUE:
a list with the following components:
x:
the centers of the x bins.
y:
the centers of the y bins.
z:
a matrix of bin counts. The number of rows is the length of the output x, and the number of columns is the length of the output y.
xbreaks:
the boundaries used to bin x
ybreaks:
the boundaries used to bin y
DETAILS:
Missing values are not counted. hist2d calculates the counts for a 2-dimensional histogram. These counts may be used in persp, image, contour or some other plotting routine. The default number of bins is computed by a variant of Doane's rule and breakpoints are chosen so that few datapoints lie on a breakpoint.
SEE ALSO:
hist , image , persp .
EXAMPLES: 
corn <- list(x = corn.rain, y = corn.yield)
image(hist2d(corn, nxbins=9, ny=9))
points(corn, col=0)


prim <- hist2d(prim9[,3:4], nxbin=30, nybin=30)
persp(prim$z/max(prim$z), eye=c(-6, -4, 3))