Often you have data points which you know are not valid for some reason and would like to exclude them from a plot. The only way I know of to do this is to edit the data file, but it can be much more difficult to distinguish those points when looking at numbers than when looking at a graph. I propose a way to exclude specific points from within Gnuplot. I imagine a new keyword such as "except" which takes a list of record numbers to exclude, unlike the "every" keyword which can only exclude data at regular intervals.
Then, the feature that would _really_ make this useful would be to have a mouse command that adds points to this list. For example, click the mouse to exclude the point nearest the cursor, or draw a box and exclude all points inside it.