title: Reference R data frame column name from string
body: <p>In ggplot(), you can use a column name as a reference in aes():</p>
  
  <pre><code>p &lt;- ggplot(mtcars, aes(wt, mpg))
  p + geom_point()
  </code></pre>
  
  <p>I'm storing my column names as strings. Is it possible to switch a string to a column-name reference in R?</p>
  
  <pre><code># This doesn't work
  var1 = "wt"
  var2 = "mpg"
  p &lt;- ggplot(mtcars, aes(var1, var2))
  p + geom_point()
  </code></pre>