I have a collection of 2d points xy.
Instead of simply plotting the points,
I'd like to plot a collection of arrows
that goes from each point to the next.
The "from" is easy of course;
the "to" is the problem.
I think I should be able to use quiver,
but I'm not getting it quite right.
x,y = zip(*xy)
u,v = np.diff(x), np.diff(y)
quiver(x[:-1], y[:-1], u, v, angles='xy')
looks like a start but does not get there.
I guessed that `scale=1` would complete
the picture, but that just showed that I