title: Options for caterpillar plots in lme4, grouping by factor to visually identify temporal trends
body: <p>I'm analyzing a large and complex data set using the lmer function in lme4. I'm using lattice and dotplot to generate caterpillar plots of my random effects. Is there a way to color code my caterpillar plot by either a factor or continuous variable that's in my data set? Have also used qqmath, I might just need help understanding how to use the "groups" argument. It would aid in a discussion point.</p>
  
  <pre><code>library("lme4")
  data(package = "lme4")
  
  summary(grouseticks)
  
  fit1&lt;-lmer(TICKS~1+(1|LOCATION), grouseticks)
  rr1&lt;-ranef(fit1, condVar = TRUE)
  dotplot(rr1)
  #We get a nice caterpillar plot of intecepts and variances by location, is there any way to color code those
  #blue intercept points by another factor, such as year? 
  </code></pre>