title: ggplot boxplot for multiple columns with a factor
body: <p>I am new to <code>ggplot2</code>, I want to plot boxplot of multiple numerical columns in the same plot, and group the boxplot by a factor from the last column the header of the dataframe is like:</p>
  
  <pre><code>id var1 var2 var3 factor
  </code></pre>
  
  <p>I know I can do:</p>
  
  <pre><code>ggplot(df,aes(x=factor,y=var1)+geom_boxplot()
  </code></pre>
  
  <p>For each variable.</p>
  
  <p>How can I plot them together with <code>ggplot2</code>? The base graphics utility <code>boxplot(df[,-c(id,factor)]</code> will plot them together.</p>