title: In R, dealing with Error: ggplot2 doesn't know how to deal with data of class numeric
body: <p>I'm new to R and haven't done any programming before...</p>
  
  <p>When I attempt to create a box chart with standard error bars I get the error message mentioned in the title.</p>
  
  <p>I used a script I found on R Cookbook which I tweaked a bit:</p>
  
  <pre><code>ggplot(GVW, aes(x="variable",y="value",fill="Genotype")) + 
    geom_bar(position=position_dodge(),stat="identity",colour="black", size=.3)+
    geom_errorbar(data=GVW[1:64,3],aes(ymin=value-seSKO, ymax=value+seSKO), size=.3, width=.2, position=position_dodge(.9))+
    geom_errorbar(data=GVW[65:131,3],aes(ymin=value-seSWT, ymax=value+seSWT), size=.3, width=.2, position=position_dodge(.9))+
    geom_errorbar(data=GVW[132:195,3],aes(ymin=value-seEKO, ymax=value+seEKO), size=.3, width=.2, position=position_dodge(.9))+
    geom_errorbar(data=GVW[196:262,3],aes(ymin=value-seEWT, ymax=value+seEWT), size=.3, width=.2, position=position_dodge(.9))+
    xlab("Time")+
    ylab("Weight [g]")+
    scale_fill_hue(name="Genotype", breaks=c("KO", "WT"), labels=c("Knock-out", "Wild type"))+
    ggtitle("Effect of genotype on weight-gain")+
    scale_y_continuous(breaks=0:20*4) +
    theme_bw()
  
  Data&lt;- data.frame(
    Genotype&lt;- sample(c("KO","WT"), 262, replace=T),
    variable&lt;- sample(c("Start","End"), 262, replace=T),
    value&lt;- runif(262,20,40)
  )
  names(Data)[1] &lt;- "Genotype"
  names(Data)[2] &lt;- "variable"
  names(Data)[3] &lt;- "value"
  </code></pre>