Allow setting min/max graph temp to something else than 0/99°C

**Is your feature request related to a problem? Please describe.**

I can't see small changes in temperature because the temp graph scale is hardcoded to whatever sensors tell is the max temperature, and then `max(this, 99)`, similarly for the min. This is not useful. Computers usually operate best within fairly small ranges of temperatures, not "disk specs" or "0-99".

**Describe the solution you'd like**

I'd like overrides in the "Graph" menu to set the minimum and maximum for the temperature scale, so the live graphs become useful & meaningful, not just all flat-ish.

**Additional context**