How many times have you done something like this?
% BEGIN draw a horizontal line
xMin = 0; xMax = 10; % the current extents of the x axis
yVal = 12;
plot([xMin xMax], [yVal, yVal])
% END draw a horizontal line
Hline and Vline
are simple but useful tools that draw a horizontal line at the given Y value from the left most extent of the axes to the right most. There is a similar command for vertical lines. You can replace the above with:
and be done with it.