Add option to set y axis limits for sensor card plots

[ast0815]

Hi, I find the plots on sensor cards to be prettier than the history plots, but because of the lack of any scale information, they are near impossible to read quantitatively. I guess that adding some sort of information about the y axis scale would mess with the design, but maybe it would be useful to just set a fixed min and max value for the y-axis instead. Then the up and down movements on the card could actually tell us something about the size of the swings of the sensor value.

[ast0815]

Case in point: Right now I cannot tell on first glance whether the living room temperature varied between 19.1 and 18 or between 21 and 15 degC.

I know I can click on the card to see the history plot, but it would add value to the sensor cards if I could see this directly.