Your own data is limited only to a specific time period, up to the year 2012 which was the highest (according to my links) year since 1999, but overall shootings are in
decline, and in fact the highest year was 1929. My links address why people "feel" (despite the data) that mass killings are on the rise.
I am instantly skeptical when a narrow window (in your case just a few years) is used to form a picture of longstanding trends. Tricks like this will make one a quick cynic-its like when proponents of the war on poverty produce graphs to show its lowered poverty-neglecting to show it was falling faster and much longer, in the years preceding the program.
Beyond that, my data is researched by a criminologist-your limited study was researched by ...mother jones.
And all superimposed on the fact that there are both more guns, and less violent crime more broadly.
Flawed study, flawed source. I hope I was able to teach you something about statistical analysis. :2wave: