title: how create histogram from data frame in R
body: <p>I want create histogram from data frame, but every time code is used I get error <code>'x' must be numeric</code>.</p>
  
  <pre><code>    df &lt;- data.frame(col1 = c(10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120),
      col2 = c(10, 21, 31, 41, 51, 61, 72, 82, 92, 104, 114, 134))
  
      hist(df)
  </code></pre>