Statistical significance line

SIGLINE plots a line of statistical significance on the current axis
sigline(xs) plots a significance line between the value in the 2D
vector xs along the x-coordinate.

sigline(xs,lbl) places a text lbl beside the significance line.

sigline(xs,lbl,h) attempts to plot the significance line above a
highest value in a graphics object with handle h. If h is not a
graphics handle, the line will be plotted above the value of h.

sigline(xs,...,vy) give a way to specify that the line be plotted above
vy which should not be assumed to be a graphics object. This is
optional and overrides h; but probaly won't ever be needed. The vy is a
value on the y-axis above which the sig line is plotted. It is only
useful for a possibly, rare occurance of a given y in h unwantedly
matching a graphic object in value.

Examples #1: Plot 5 random points and add a significance line between
point 2 and 4.
plot(rand(5,1))
sigline([2,4])

Examples #3: Plot 5 random points as bar chart and add a significance
line between point 2 and 3 and print p=0.05 beside it.
bar(rand(5,1))
sigline([2,3],'p=0.05')

Examples #4: Plot 5 random points and add a significance line between
point 2 and 4 and specify the handle to be used to obtaine the value
above which the line will be plotted.
h=plot(rand(5,1))
sigline([2,4],[],h)

Examples #5: Plot 5 random points and add a significance line between
point 2 and 4 and a value above which the line will be plotted.
plot(rand(5,1))
sigline([2,4],[],0.9)