The graph does not show growth increasing linearly, as you say. What it shows is that the rate of growth is increasing linearly, which indicates that there is an acceleration in how fast solar power capacity is being created. Thus, the growth of installed power plants will in fact be increasing quadratically.

In layman's terms, quadratic growth is often referred to (incorrectly) as "exponential" growth. If the article included a graph of the total solar power capacity over time, it would indeed appear to be increasing "exponentially."