x1, y1, val1
x1, y2, val2
x1, yN, valN
x2, y1, val(N+1)
x2, y2, val(N+2)
x2, yN, val(2N)
...and so on...
Once you've done this, here are the magic Gnuplot commands:
splot 'mygriddata.txt' with lines
set cntrparam levels discrete 19.7, 20, 20.5, 21, 22, 23, 24
At this point, you should click on the graph and spin it around until it looks nice. Then you can save it to disk as a PNG (for better quality, use PS output):
set terminal png
set output 'plot.png'
If you want it even nicer, you can try:
set pm3d at sb