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.

Files opened from command line would create a dialog but never display it, keeping a handle to it open. This interacted with a strange wx bug to cause a hard crash if a file in the same directory as the active directory of the dialog was edited with vim. This commit should prevent these crashes.