Ignore stupidly low temperatures (< -20C)
Ignore stupidly low temperatures (< -20C)

Only plot temperatures above -20C.  When you unplug a sensor, the
BlueTherm's reading for that sensor decays over a few seconds down to
-300C before returning Err.  During those few seconds you end up getting
spurious reading like -180C, which we obviously want to ignore.

This patch also speeds up start time by only pre-loading the last 600
results.  cur.fetchall() was taking 50s to run, returning 18k rows.
Naive Python is not fast.

edwardspeyer/bluetherm-toolkit,edwardspeyer/bluetherm-toolkit,edwardspeyer/bluetherm-toolkit