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...to use filled.contour() because of this. Note that filled.contour() calls .filled.contour(), which calls: + .External.graphics(C_filledcontour, x, y, z, levels, col) Also note that I had a similar plot with a similar problem. It contained the following line: + polygon (xsub, ysub, border=NA, col=colstr) But the problem was corrected after changing it to: + polygon (xsub, ysub, border=colstr, col=colstr) I'm assuming that an equally simple change to C_filledcontour, could correct the problem above. [[alternative HTML version deleted]]