The maximum number of total data points in a plot is 256,000. However, I
have setup plots with 1.5 million data points. Excel does not complain
about this, but I'm guessing it is still only showing 256,000 points. The
thing is, I can't figure out which 256,000 it is showing. I don't think it's
just the first 256,000 because the data is sorted and it would look abruptly
cutoff after the 256,000th point.

I'm using a scatterplot if that helps. The plot looks correct, with
gradually decreasing density towards the outer edges of the plot. But if
it's only showing 17% of the data points, is Excel intelligently filtering
the data evenly?