(Cherry-picked from d56bcd8e, the
experimental graph coloring branch)

The variable 'dot' in GraphMeterMode_draw now means "maximum number of
dots per value (column) in graph". The old meaning was "amount of value
that is to be represented by a dot" and was always a fraction. Due to
a limitation in floating point computing, if GRAPH_HEIGHT were not a
power of 2, then rounding errors will occur on numbers like (1.0/3).
(Currently GRAPH_HEIGHT is 4 and so no precision loss.) 'dot' was used
as a divisor, and it's "division by a reciprocal". We change that to
simple multiplication.