Allow plotting weight in Graph.plot

[martinfleis]

A side note - I also realised that we can directly plot weight. Neat.

Originally posted by @martinfleis in #627 (comment)

In explore, we get ability to plot the actual weight on a map for free but with static plots, we need to get matplotlib colormap, retrieve the colours and pass them to the LineCollection ourselves. Might be worth exploring if there's an appetite.

[jGaboardi]

xref #562