Change number of ticks based on plot size

Original [issue 38](https://code.google.com/p/plotinum/issues/detail?id=38) created by eaburns on 2012-06-21T12:20:38.000Z:

It's nice to have the number of tick marks computed by DefaultTicks change based on the size of the plot.

SPT (code.google.com/p/caml-spt) uses 1 major tick every 4cm, but no fewer than 2 major ticks, as its hint for how many ticks to attempt. IMGO, SPT sometimes uses too few ticks. Maybe 1 tick every 4cm, but no fewer than 3 would be good for Plotinum.