[R] dotted contour line for negative values?

[Bill Simpson]

I have a contour plot where the surface can take on positive and negative
values.  I would like to use solid contour lines for positive values and
dotted contour lines for negative values. I saw a plot like this produced
by S-Plus before.  Is there a way to do this with R? If not, can anyone
suggest a way to make interpretation of the contour plot easier? (Right
now, it is not clear if a set of "tree rings" in a plot indicate the
presence of a peak or a trough)

I am thinking that it might be possible by making one plot of the positive
contours with lty="solid", add=TRUE, then superimpose the negative
contours with lty="dotted". The snag is: how to get only the positive or
negative contours?

Thanks very much for any help.

Bill Simpson

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