While the graphic representation of my thermostat ‘on time’ is very pretty, I have no idea what the rainbow of colors is representing. The resulting graph is the result of the hvac_action entity that has been further converted to hh:mm from the fractional hours result.
When a sensor’s value does not have a unit_of_measurement specified then the graph looks like in your case.
Specify a UoM → get a nice graph with curves.
So you’re wanting to graph time over time? Not trying to be cynical, but what good is that? It seems like it would be much more interesting to graph literally anything else over time.