add information to data based on a fitted model
fortify.merMod(model, data = getData(model),
    ...)fitted model
original data set, if needed
additional arguments
fortify is a function defined in the
  ggplot2 package, q.v. for more details.
  fortify is not defined here,
  and fortify.merMod is defined as a function
  rather than an S3 method, to avoid (1) inducing
  a dependency on ggplot2 or (2) masking
  methods from ggplot2.
  This is currently an experimental feature.