Some ranging fixes for volatile data

[Dima K. <gn...@di...>]

Hi.

Recently Ethan committed some fixes to make autoranging refreshes work
for volatile data in commit titled

 Partial axis ranges, e.g. [*:MAX] or [MIN:*], were not being handled
 correctly when refreshing volatile data.

Here are two patches to fix some more related issues. The first patch
applies similar logic to 3d plots. A test script to show the behavior
being fixed:

 set zrange [:3.5]
 splot '-' using 1:1:1 with linespoints ps 3 pt 7
 0
 1
 2
 3
 4
 5
 e
 refresh


The second patch handles the case of degenerate ranges. Demo script for
2d plots:

 plot '-' with linespoints
 0 1
 0 2
 refresh

and 3d plots:

 splot '-' with linespoints ps 3 pt 7
 0 1 2
 0 3 4
 refresh