Use Numbers instead of points

[gnunicorn]

Suggested by a group of participants: to understand the loop in a loop better, counting up a number and draw that instead of a point. It visualises better what it going on and where what happens (and overlapping is more obvious).

[xMartin]

Actually the same issue as #30 but probably much easier to implement.

[grncdr]

another approach: I told them to put console.log('drawing a circle at', x, y) before each call to circle and that helped enormously.