Provide an option to render arrows between points instead of lines
For example, an arrow would be drawn from each element in a series to the subsequent element.
One arrow at the end.
Now I come to think of it: an arrow between the first point of Series 1 and the first point of series 2 (and between second points, third points, etc.) would be very useful too. It could indicate the development between two years.
It would probably be best implemented as a small plugin. Where would you like your arrows? In the middle of the lines, at the start, at the end? One arrow or multiple arrows?
This would be useful with score cards where you can connect a number of scores (through time)
See http://jsfiddle.net/kzoon/uQMCA/2/ for example