How the hell would they get accurate crime stats from 1200? A year so long ago that I feel the need to suffix it with "AD".

Homicides are easy to document, and those are the stats used in the graphs. It's other crimes that are difficult to compare.

Find an old source saying "Blimey, we hath hadd 200 of thee black cryme of murder thys annus!" and cross-reference it against recorded population at the time, and there's your homicide rate.