Relating energy demand
to degree days

The line that you plot becomes your target or yardstick performance characteristic.

If there is a wide spread in your scatter, and assuming that you have reasonable confidence in your meter readings, you can safely bias the line towards the lower envelope of the plotted points.

This is because over-consumption is far more likely, in most cases, than under-consumption. Hence you would not expect the spread of error to be equally either side of your target line.