The diagram is about heating fuel choice according to the construction years, there are two lines on the diagram, the grey one is natural gas, and the black one is electricity. The two lines were intersect at the time point 1990-2010.
The natural gas line was started at 60% in the beginning, but it doen not change during before 1950 till 1969, during the 1969 till 1989, it was decresing till 45%, after 1989 till 2010 was keep the line change in 45%. For the electricity line, it started at 15% in the beginning, but it was increasing in every time period, during the 1950-1969, it was increasing till 25% , later during the 1970-1989, it was increasing till 35%, the last increasin g of this diagram was in 1990-2010 this period, it was increasing till 45% and intersect with the natural gas line.
In conclusion, the two lines of electricity and natural gas users percentage were showing the users increase and decrease in diffrent time period during before 1950 till 2010, it obvious to see the changes on this diagram.