log plots: label minor ticks


    > returns a NullFormatter (you would think it would return a
    > LogFormatter). This made me think that both major and
    > minor ticks were handled by the major formatter (which
    > distinguishes them based on it's internal isDecade() call).

Well, I use a NullFormatter because minor tick labels are turned off
by default. Otherwise the labels could get quite busy. Note that
there are 4 players here

  major and minor tick locators and
  major and minor tick formatters

The minor tick locator is a LogLocator which recognizes the subs --
the minor tick formatter by default is a NullFormatter. This places
tick lines and empty tick labels.