Answer
Climate: 2011 may have been unusually warm, causing more people to think of solar power as a cost efficient investment. In 2012, the temperatures may have begun to cool, and 2013 may have been unusually cloudy, causing people to doubt whether solar power could meet their needs.
Economy: A booming economy in 2011 may have given people the disposable income to invest in solar power. A dip in economic health during 2012-13 may have left people less money to invest.
Media: The media can affect people's actions and purchases. A series of news stories in 2012-13 about scandals in the solar industry could cause new installations to drop.
Work Step by Step
Line graphs show us trends or patterns. If you draw a straight line from 2009 to 2014, you will see that most years fall on or near the line, meaning there was a fairly steady increase in installations each year. This tells us that the number of installations in a year is not random; we can expect a slight increase over the previous year.
The exception are the years 2011-2013. Because the number of installations is not random, we look for factors that would cause them to fluctuate temporarily, in this case to spike in 2011, then drop in 2012-13. Some of these factors could include climate, the economy, and the media.
