This gives a graphical view of your previous drill performance. The information available here is more detailed than the summary shown in the Statistics log window. Success rates for each character are shown. If you want to view your progress for each character over a time period then go to the Statistics, progress window.
Right click in the chart area to pop-up some display options for the chart. Left mouse-drag on the white parts of the chart to move it horizontally. Left-click on a bar to pop-up details of the bar data. You can display results for the last code session or any saved log entry.