Plot persistent vertical and horizontal line

VHLINE Draws a horizontal or vertical line that persists through pans and zooms.
VHLINE('x',val) draws a vertical line on the current axes that intersect the x axis at the location specified by 'val'.
VHLINE('y',val) draws a horizontal line on the current axes that intersect the y axis at the location specified by 'val'.

VHLINE(_,linetype) defines the aspect of the line. All the usual
specifications used for the "plot" functions are valid, including key-value pairs

H = VHLINE(_) returns the handle for the line

Example
vhline('x',10)
vhline('y',1,'r--')
vhline('x',5,'Color',[.7 .7 .7],'Linetype',':')

The HandleVisibility property of the line object is set to "off", so not only does it not appear on
legends, but it is not findable by using findobj. Specifying an output argument causes the function to
return a handle to the line, so it can be manipulated or deleted. Also, the HandleVisibility can be
overridden by setting the root's ShowHiddenHandles property to on.

The Tag property of the line is used by the function to maintain the
line visible. Modify of overwrite it will cause this behavior to stop.
Any Tag specified by key-value par is overwritten.

The ActionPostCallback properties of the Pan and Zoom tools of the
current figures get modified by this function.

by Luca Amerio
luca.amerio@polimi.it
27/01/2016