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.

Conflicts:
	pronterface.py

This callback is useful to broadcast the last print line sent in the form of
the parsed gcoder.Line object. We can then augment the gcoder.Line to do
awesome stuff, such as updating the print status in the 3D viewer.