title: Using sorting variable in doBy functions
body: <p>I have noticed a strange phenomenon that I'd like a sanity check on.</p>
  
  <p>When I assign a letter to the name of variable (e.g., z<code>&lt;-Type or z=Type</code>) where Type is a variable in a dataframe, yf, I can use <code>z</code> as a sorting variable for functions like making a linear model (e.g.,<code>aa=lm(y~z)</code>) or making boxplots (e.g., <code>boxplot(formula=y~z)</code>, but <code>z</code> doesn't work in the doBy functions. In other words <code>r=summaryBy(y~z, data = yf, FUN= c(mean, var, mystderr), na.rm=TRUE)</code> doesn't sort <code>y</code> by <code>z</code>, but <code>r=summaryBy(y~Type, data=yf, Fun=c(mean,var, mystderr), na.rm=TRUE)</code> works like a charm.</p>
  
  <p>Does this make any sense? My program works just fine when I call the sorting variable directly, but I wasted a fair amount of time getting to this point.</p>