xtgraph: summary graphs of xt data

xtgraph produces summary graphs of xt data, by time and by group. It is very flexible, allowing means based on any power or 3-parameter log transformation, and error bars for SE, CI, SD and Reference Range, as well as medians with IQR. Normally, points are estimated separately for each level of t and group. However, a model option will take values from the last model fitted. This allows for linear and non-linear effects and displays interactions. The main illustration deals with data about Vitamin C and E supplementation.