Custom - Custom Graph - How to?

[paul402]

To regulate the humidity in my cellar I calculate absolute humidity for the cellar and outside. When the absolute humidity outside and the temperature outside is lower then the cellar is ventilated. It works ok although my absolute humidity is displayed as "Temperature" on the graphs.
I can live with that.
Using custom graph I can show the temperature of the cellar+outside and my absolute humidity values for both.
But I would like to show the relative humidity as well.
The only thing I can think of is to create a fake temperature variable and assign relative humidity to it, then display it.
Is there a better way/something I missed?