The problem is that there is no good "default" constructor for
ColorTable. The previous default constructor created an empty color
table, but that would be confusing if someone actually tried to use it.
We could set ot to the default preset, but the default preset uses the
diverging color map, which could foul people up if they actually want to
edit or create their own color map. Instead, force the declaration of
ColorTable to indicate what you plan to do with it.