Tested on both wx2.9 and wx2.8, shouldn't bring any regression.

Temperatures were being plotted upside down.

Should also run slightly faster

Tries to determine which sensors are active and keep those within the
bounds of the graph. For instance, if the bed is disconnected, zooms to
show only the extruder, allowing sub-degree changes to be shown.

Initial steps towards rescaling the graph to better display detail.
Not completed.