Gas consumption and insulation
Use this diagram to explain how interpretation is easier if the regression lines in all groups have the same slope — the difference between the expected response in different groups is simply the difference between their intercepts, irrespective of the value of X.
Select Parallel lines from the pop-up menu to fit parallel lines by least squares. Based on these lines,
Gas consumption is (6.717 - 4.922) = 1.795 lower after insulation, regardless of the outside temperature.
You should also mention that parallel lines are probably not appropriate for these data. You would be surprised if the effect of insulation was not greater when the outside temperature was low!
Always think about the context.
Data were collected in the 1960s at a house in south-east England about weekly gas consumption (in 1000 cubic feet) and the average outside temperature (in degrees Celsius) for 26 weeks before and 18 weeks after cavity-wall insulation had been installed. The house thermostat was set at 20°C throughout.