We had some great feedback on our new SPC charts over the last couple of weeks, including someone who wanted us to revert to computing the target and mean lines from all 100% available data, rather than the 50% we were asked to change to earlier.
It's not uncommon to have conflicting customer demands like that. No worries, here's the solution:

Screenshot

This is the default, with the point to which the means etc are computed highlighted with a green circle (it's the March 2019 point here). You can click any point to switch the compute up to and including that point. The point's details are shown in the legend and the mean plus process limit lines adjust in real time as you'd expect.

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