get x/y-axis max value

[TimmWesselath]

Hi.
I want to draw a horizontal and a vertical line in my graph. I want the lines to go all the way to the edge of the graph and stop at the point where they cross. I have some dynamic data in my graph so I can't have the lines be a set length.
I was wondering if there was some way to get the max values from the x- and y-axis.