"activepositionproperty" and the gnuplot backend

[Ben Abbott]

I've prepared a changeset for the Gnuplot backend that treats the 
"activepositionproperty" is a manner similar to Matlab.

There should be no impact on the FLTK backend.

With this changeset, the instances of plot labels being clipped or cropped for 
printed output can now be avoided by setting the "activepositionproperty" to 
"outerposition".

There are differences as compared with Matlab.

For Matalb, when the "activepositionproperty" is "outerposition" (the default) 
the plotbox position is normally determined by the "position" property. 
However, if the axes/tick labels begin to move beyond the "outerposition" 
property, the plotbox position is adjusted to ensure the axes/tick labels are 
fully visible.

In the changeset attached, the "if labels don't fit inside the outerposition 
property, then adjust the position property", is not handled. Instead, 
"outerposition" property represents the bbox of the plot box and labels. This 
is a necessary feature when using the gnuplot backend. It is also one I prefer 
since the Matlab approach occassionally confuses me.

The manner by which Octave behaves when the "activepositionproperty" is set to 
"position" remains as it has been.

Another difference is that Octave's colobar sets the "activepositionproperty" 
to "position", while Matlab's implementation does not. This means that Octave's 
colobox's behavior has not changed.

I've tested this changeset with gnuplot 4.2.6 and 4.4.

Since several changes have been made to the backend in the last few days, I'll 
do some more testing before committing.  In the meantime, if anyone has any 
comments or would like to test drive this change, I've attached the changeset 
below.

Ben

changeset.patch
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