Test Trend Chart and Statistics Chart shows "0" failed tests but there were failures during the days displayed in both chart. Please see the screenshot, which shows dashboard's Test Trend chart compared to "Build History" and xUnit's "Test Result Trend" chart, which shows two failures...

I'm experiencing also another problem (maybe related), which shows only small amount of days in Test Trend Chart. I've posted about it here:
https://issues.jenkins-ci.org/browse/JENKINS-35268