[Python] White lines in matplotlib's pcolor

In some pdf viewers such as Preview on OSX, plots made with matplotlib's pcolor have white lines (see image below). How can I get rid of them?

The source code is very simple (choose any data for x,y,z):

import matplotlib
matplotlib.use("pdf")
import matplotlib.pyplot as pl
pl.figure()
pl.pcolormesh(x,y,z,cmap="Blues",linewidth=0)
pl.savefig("heatmap.pdf")




Continue reading...