Regression tools now filter based upon group selection

Addresses issue #1558 .  The issue here wasn't how regressions handle multiple data sets, but rather how the data was filtered.  Before the regression is computed, data clipping is performed on all data, regardless of whether or not it is currently being displayed.  I added a small procedure to filter out all data points that are not currently being displayed, such that only visible data is used when computing regressions.