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.

This allows the white background to show through the Graph,
fixing the background problem in #359

@iXce's change in 94d69f79 was on the right track, but
has to occur earlier in the draw cycle.

This should fix the weird graph background for #359

This fixes bug #359

Changed order to draw the target line first, and then the actual line so this line is shown above the target line.
Graying out graph when it's offline.