1) Your temperature reading is much finer in scale than the target temperature can be, so it looks like the error is much larger than it is.
2) You're cooling, not heating, so temperatures can float upward on their own and cooling is what is bringing the temperature reading down.
I'd be curious to see the actual on/off time of your A/C versus the input reading. Are you using PID control at all or is this too large a system to control like that?
1) The target temperature actually is subject to change as often as I take a temperature reading, but it just happens to alternate between a few predefined (in google calendar) states. I could make that line smaller/faded/etc to make the actual temp stand out a bit more
2) true :) is the name of the project confusing, since it's currently cool-or-not ?
What would you like to see with regards to the on/off time vs. the temperature? I was thinking about a faded background highlight when the HVAC was on (blue for AC, red for heat). Is there something more that would be interesting?