detect single-point arcs and draw as points

[ghorn]

Right now it expects lists of arcs: [[(t0,x0), (t1,x1)], [(t2,x2), (t3,x3)]]
When it gets an arc with one point in each arc like [[(t0,x0)], [(t1,x1)]] it will draw nothing.

This should be efficiently detected and should be drawn either as dots or one line