Below is a graph of Calvert Equity Fund Class 1 (CEYIX) compared to the S&P 500 (.INX) in Google finance with "Max" clicked (so it starts Jan 14, 2000). Calvert is kicking ass until Dec 2015, when there's a big drop. What was that drop?
Check to see if there was a special dividend/distribution in 2015 that isn't being accounted for in the graph that you posted.
You could cross reference this by looking at their year by year performance. My guess is that the annual performance for 2015 will not reflect this approximate $10 drop, reinforcing the idea of a dividend/distribution.