TODO
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1) Allow user to specify the steps
2) y_max_value => max_value
3) y_tick_number => steps
4) Add more colours, and the ability to specify own
5) Allow choice of start position as per a clock face (0 .. 12) or via the
   angle of rotation (0..360), both starting from top dead-centre.
6) Allow more point types (e.g. cross, unfilled circle, unfilled square,
   unfilled triangle and unfilled diamond)