luci-app-statistics: Voltage graphs - AC and DC

The Voltage graph combines Battery, Input Voltage and Output Voltage.
The Y-Axis scale masks changes in Input/Output voltages over time.
This patch splits the graphs into 2 graphs.
This makes it possible to see variations in AC Input/Output voltages.

Signed-off-by: Bob Meizlik <bobmseagithub@squakmt.com>