To plot estimated coefficient functions from different models together, one
can first combine the data frames returned by coef, and then call
plotCoef, for example, plotCoef(rbind(coef(fit1),
coef(fit2))).
To specify the time range of the plot, one can either utilize the
ggplot functionality, say plotCoef(coef(fit)) + xlim(2, 10);
or manipulate the data frame first before calling plotCoef, e.g.,
plotCoef(subset(coef(fit), Time > 2 & Time < 10)).