Create a straight faint dotted/dashed line through y=0

Question

I want to be able to draw a faint dotted or dashed line through y=0 to make it more obvious when the plotted line goes below 0. If possible id like the whole area of the graph below the y=0 line to be lightly shaded to show even better when the plotted line drops below zero.

Here is just an example of a type of graph that i would want to shade/draw a line:

http://s27.postimg.org/v94uey56r/Rplot02.png

There are lots of different graph types that i would like to do this to so it doesnt have to be specific, just aslong as it works so i have a general idea of what to do is good enough :)

thankyou

Solution

How about abline(h = 0, lty = 2)?