Average MPG on Consumption Screen

I'm a little confused about the 'Average MPG' readout on the consumption screen. I see that the yellow bars clear when I stop and then re-start the car. I also see that the average MPG and mileage readouts continue even though the yellow bars clear.

So the question is: Does the system average out the MPG over the entire tankful of gas? ... or every 30 minutes as the consumption screen shows the graph over a 30 minute period?

I have been experimenting by driving very smoothly and carefully (pulse and glide) and then trying a more aggressive style. However the 'Average MPG' doesn't show much change over the course of 30 minutes of driving. It seems to keep the average MPG pretty constant over the entire range of the tankful of gas.

Thanks! Marvin