On my Fedora PC GNUPLOT 4.0 is installed which does not permit (or at least I did not manage) to overlay contour lines on top of a color map. The newer release (i.e. 4.2 )of GNUPLOT allows to do this, by running a tricky procedure described here. As this version is installed under Cygwin on my Laptop I am now re-creating the plots for the hard-copy version of our ionosphere tomography paper.
Here’s a sample, which makes Matlab look old!