[Kst] [Bug 110734] Ability to mask datapoints for fits and other operations etc.

[Barth Netterfield]

IIRC, the way this is done in Origin is a spreadsheet mode.  The unwanted 
points are indicated somehow, but are still plotted - perhaps in a different 
colour.

But we need a way to handle 10^lots data points in a spreadsheet.  Rumour is, 
QT4 will support such things.

Then we need to figure out how to handle it internally.  It may be the same 
mechanism as will be needed to support 'fit to visible points only'...

This should not be done in a hurry - we need to think it through better.