[R] order panels in xyplot by increasing slope

I am creating a few dozen multi-panel time series plots using lattice graphics in the lme4 package. Each panel in a given plot represents a tree. each multipanel plot is a particular treatment. Here's my issue: when you use xyplot() to plot this data it orders the panels alphabetically. I would

[R] Error: contrasts can be applied only to factors with 2 or more levels

I have a dataframe that I am attempting to analyze using the lmList() function in package lme4. I'm using this funciton to select which parameters in this model wqarrant random effects. I have a subset of the data below. I input data and change class of necessary items so that I have several fac