Allow to define absolute scale/color mapping

[mleiber]

Allow to optionally define the temperature range with absolute boundaries (e.g. allow to set an explicit range from 5 to 30 degC).

Along the same lines allow to define the color mapping using absolute values for low and high temperature.

Clip too low or too high values. There might be means to visually indicate that clipping happens, e.g. 3 dots or the like at either end of the bar. Shouldn't be too intrusive to not compromise the nice look this UI element has.).

The use case is to be able to have multiple graphs visible at the same time and be able to directly compare them. I use it in a tabbed card for 3 locations that I monitor and compare frequently.