An example of using the facet_wrap function to arrange the panels for a single conditioning variable on multiple lines.
Modern Applied Statistics with S (Statistics and Computing) W.N. Venables, B.D. Ripley
Interactive and Dynamic Graphics for Data Analysis: With R and GGobi (Use R) Dianne Cook, Deborah F. Swayne
Mixed-Effects Models in S and S-PLUS (Statistics and Computing) José Pinheiro, Douglas Bates