Sometimes it is required to use two Y-axes in the same graph. The method to achieve the goal is depended by the basic graphic package, such as ggplot2, lattice. Because ggplot2 is my first choice, so I’m trying to get a solution for ggplot2.
kohske’s solution is the only one I can find right now.
http://rpubs.com/kohske/dual_axis_in_ggplot2
http://rwiki.sciviews.org/doku.php?id=tips:graphics-base:2yaxes
http://robjhyndman.com/hyndsight/r-graph-with-two-y-axes/
http://evolvingspaces.blogspot.de/2011/05/multiple-y-axis-in-r-plot.html
http://stackoverflow.com/questions/6142944/how-can-i-plot-with-2-different-y-axes-in-r/