I think you can invert the values in Grafana itself on the Display tab as a custom series parameter like you would do to assign a series to the right y-axis. I experimented with it trying to come up with a graph with my RFM69 gateway transceiver's temperature with the RSSI so I can see if temp is affecting my signal strength (it is). In the end leaving it not inverted worked better for me. I'll post some screenshots when I get home, though they are really boring.
Edit: My graphs so far. NOTE: I probably will never use Grafana's dashboard so this is how I'll see them in my day to day use.
Inside temps verses outside temps and switch indicating when the central heating fan is on (I'm hoping to prove to myself that running the fan does equalize the temps across my three floors (NOTE: I'm still wiring and coding up the sensor for the basement).
Inside Humidity verses outside humidity. This will become more important in the winter as the winter humidity here stays between 15% and 20% and I like to keep at least the bedrooms at 35%. A whole house humidifier is in the plans at some point.
Light levels. More sensors are in the works. Not sure exactly what I'm going to do with this beyond using the graph to help determine when to turn on/off the lamps instead of the weather.
I've created an RFM69HW gateway on a Raspberry Pi and I created one so far "Uber Sensor" from the Uber Home Automation Indestructible. This graph is telling me the temperature reported from the transceiver on the Gateway verses the signal strength reported detected when the sensors communicate with gateway. I admit I don't know much about wireless but an RSSI of -95 to -85 seems pretty poor to me and it fluctuates. I'm hoping I'll be able to see some correlations on this chart (e.g. I've already seen signal strength goes down when the gateway temp goes up). This is probably my most actively used graph right now.