Chart::Math::Axis implements in a generic way an algorithm for finding a
set of ideal values for an axis.  That is, for any given set of data,
what should the top and bottom of the axis scale be, and what should the
interval between the ticks be.

The terms top and bottom are used throughout this module, as it's
primary use is for determining the Y axis.  For calculating the X axis,
you should think of 'top' as 'right', and 'bottom' as 'left'.