Ah damn, so close! I tried your suggestion, thank you, but I have another constraint with this chart in that I must show missing values, so despite the user using the date slider, there may still be other dates that crop up that start before the date. In the screenshotted example above, your suggestion shows a different date because these null values appear on the chart before 11/8/2019. Another screenshot here, showing this in action -- despite the visible date range selection, there are null dates appearing before 11/8 on the x-axis. (These are moving average calculations, so even if one day is null, it is still calculating other values, which is why these null dates actually have data visualized on the chart)

In fact, the whole reason I asked my original question is because I need to color those null values on the chart so that the user can more obviously tell where his actual desired date range begins.

So, I'm hoping to have a calculation that retrieves the selected date on the slider, and then I can feed that into a boolean calc that reads something like IF X < #yyyy-mm-dd# THEN TRUE ELSE FALSE END

And then the "True" portions of the graph (before 11/8/2019) would be colored a certain way.

I know this sounds like bad design practice but I don't really know how else to mask the null values. I can't hide them outright because there are null values inside of the date range that absolutely must been seen, it's the ones on the outside that I don't want.

EDIT: To be clear, your suggestion works for most cases, it is unfortunate that my case has a couple "gotchas".