title: plot() does not show all diagnostic plots for lme/lmer
body: <p>When using the lme/lmer function, I cannot get R to display all 4 diagnostic plots (res vs fit, normal-QQ, scale-location, res vs leverage) with par(mfrow=c(2,2)) and plot().
  I just get the res vs fit plot and nothing else.
  I have no problem when using the lm function.
  Does anybody know how to do this?</p>
  <pre class="lang-r prettyprint-override"><code>library(lme4)
  m0&lt;-lmer(hematology~Treatment*day+Gender+(1|ID),data=long,na.action=na.omit,REML=FALSE)
  par(mfrow=c(2,2))
  plot(m0) 
  </code></pre>