A percentage is a ratio of two numbers, in this case you are trying to compare
^People who engaged in violence and were convicted at one protest^/~People who were at that protest~
with
^Number of protest events that became violent^/~Number of protest events for this cause~
To compare that you will have to wait a few years for all of the court cases to wind down. Hope that helps.
EDIT: Format
Would be funny if the last panel had George Lucas in the turbolift with a knife