The graph is from Danny Hayes, writing over at Wonkblog. He concludes:
But the media tend to be reactive, “indexing” their coverage according to the political debate happening within Washington. The surest way for an issue to stay in the news is for politicians to fight about it. (It doesn’t hurt when the president mentions it in an inaugural address.) When political elites publicly push for legislation, as the Democrats are doing now, journalists take notice. But if policymakers stop publicly arguing over gun control — either because legislation appears moribund or because debates over fiscal and spending issues crowd it out — the news media are likely to stop caring about it, too.
The full post is here.