Calculate contour lines for a given set of data.
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x, y 
locations of grid lines at which the values in 
z 
a matrix containing the values to be plotted ( 
nlevels 
number of contour levels desired iff

levels 
numeric vector of levels at which to draw contour lines. 
contourLines
draws nothing, but returns a set of contour lines.
There is currently no documentation about the algorithm. The source code is in ‘R_HOME/src/main/plot3d.c’.
A list of contours. Each contour is a list with elements:
level 
The contour level. 
x 
The xcoordinates of the contour. 
y 
The ycoordinates of the contour. 
options("max.contour.segments")
for the maximal
complexity of a single contour line.
contour
.
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