o.e.nebula.visualization.xygraph: Changes to autoscale algorithm

This adds code that prevents one range from getting 'cut' while the
other was being extended, which resulted in flickering between ranges.
o.e.nebula.visualization.xygraph: Changed name of variable

Chose more representative variable names:
   tempMin/Max ==> dataMin/Max
       min/max ==> AxisMin/Max