overline(xx, y, text), where xx is a 2-vector and y a scalar, draws a horizontal line just above (xx(1), y) to (xx(2), y) and places the given text over it.
overline(xx, yy, text), where both xx and yy are 2-vectors, draws the line just above the larger yy value and extends a vertical line down to the smaller yy value.
The whole thing is displaced by a distance overlinedist from the data, and the text placement uses the absolute value of textdist.
If overlinemin is non-zero, a vertical line is drawn on both ends.
overline(xx, y) or qoverline(xx, yy) is permitted: no text is drawn.
Optional arguments datadist, textdist, and minlen override global settings.