Often we need to draw a vertical or horizontal line across the plot, to mark the boundaries or min/max value of the function. It’s not obvious at the first glance how to do it, becase the X and Y limits can be different, and also we need to handle zoom commands. I’m now using the solution, based on graph2d.constantline which was found here

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   GRAPH_COLOR=[.7 .7 .7];
   hx = graph2d.constantline(t_start, 'LineStyle',':', 'LineWidth',2, 'Color',GRAPH_COLOR);
   changedependvar(hx,'x');
   hx = graph2d.constantline(t_start+duration, 'LineStyle',':','LineWidth',2, 'Color',GRAPH_COLOR);
   changedependvar(hx,'x');
   hy = graph2d.constantline(magnitude, 'LineStyle',':','LineWidth',2, 'Color',GRAPH_COLOR);
   changedependvar(hy,'y');

It looks like this, and it also correctly handles zoom in and out commands.

UPD 03.04.2015 Unfortunately this does not work in Matlab 2014b or higher. Try this solution .