TODO
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1) Add more colours, and the ability to specify own
2) Allow choice of background colour
3) Make legend box optional
4) 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.
5) Allow more point types (e.g. cross, unfilled circle, unfilled square,
   unfilled triangle and unfilled diamond)

