log axes labels for the FLTK backend

[Ben Abbott]

Michael Godfrey and I have been discussing the strings used for the axes labels 
for log plots.

He pointed out that some number are in %f format and others in %e, often for 
the same axis.

We'd like to propose a change so that the labels are formatted in a consistent 
manner for each axis.

The attached changeset will use %f, unless %e is shorter. In the latter case, 
all ticklabels will use %e. We'd also considered just using %e for every case 
(simpler and more uniform).

Does anyone have a preference / objection?

With the patch applied and using the FLTK backend, the changeset can be tested 
by ...

        figure (1)
        clf
        loglog (logspace (-5, 5, 11))

        figure (2)
        clf
        loglog (logspace (-2, 3, 11))

Ben

changeset.patch
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