luci-app-statistics: change nut voltage graph combines view.

This change splits the graph into two graphs like in apcups and makes it so
possible to see variations in AC Input/Output voltages. Battery charge and
load are indepentent variables. This change also overlays them and display
them as simply two lines. Battery temperature is also displayed as a line now,
as an area under a temperature line makes no sense. Also removed some empty
lines.

Closes: #2417

Signed-off-by: Fabian Schmid-Michels <mail@wohnheimnetz-bielefeld.de>
Signed-off-by: Florian Eckert <fe@dev.tdt.de>