Karl Ratzsch - 2016-12-19

I guess this is to be expected: With "x2", it is pretty clear what you want: Just add the top left and right corners to the end of your list of datapoints, and fill the polygon that is formed.

With "y=...", gnuplot needs to look for the points where the line connecting your data points crosses that vertical line. That's easy when all the points come in ascending order of abscissa values. If not (like in your case), it's just not clear what the intended (by you) behaviour should be, imo.

The help could probably explain this a bit more. But really what you want should be done with the "closed" parameter. The options that specify axes or lines parallel to the axes were not meant to be used to fill polygons, I believe.