Around 2:00 on March 29th, Bernie Sanders announced he'd be holding a rally in Pittsburgh on March 31st. This map tracks mentions of @Burghers4Bernie, the local Sanders volunteer group, before and after the rally announcement. You can see the tweets explode after the rally announcement, and I'm shocked at how wide their reach is (try zooming out).
I also included another layer that shows retweets by count. You can select that by clicking the "visible layers" button above and toggling the layers.