Improved flexibility of plot_exps.py script.

* No longer needs an X connection to render. This also vastly increases
performance.
* If too many configuration values are plotted, a key with color=column1,
line=column2, marker=column3 etc is not created. Instead, each combination
of values is given its own line/color/marker style and plotted, and each
line has an entry in the key. Ugly, but better than nothing.