In a prior SCADA software that we used, we were able to create two vertical lines on charts. A single click would put the normal line on the chart and give you the values of the pens at that point in time. A double-click elsewhere on the chart would put a second line on the chart and the pen readouts would switch to showing the difference between the two points in time. The time difference was also computed and displayed. This was very helpful in quickly determining how long an event lasted, or how much a reading changed in a given amount of time. The two lines were cleared by clicking the labels that appeared below the chart.

I think something similar would be useful for the charts in Ignition. Some similar mechanism to trigger the second line like a shift-click could be used. Ctrl-clicking or Alt-clicking the chart could clear the lines.