plotmark

Identifies lines in a figure by a single large marker.

When doing black and white figures with several curves, it is sometimes
difficult to uniquely identify each curve, since the number of different line
styles is quite limited.
This function implements one possible solution, which consists in
marking each curve with a single symbol located somewhere along the curve
(as opposed to using the symbol repeatedly at each datapoint)
The way this is done is by actually creating two graphical objects,
one for the line, and one for the symbol.
A trick is used so that only one item shows up in the legend,
with the correct identification.