title: error argument "df1" is missing, with no default
body: <p>a friend of mine is working with the r language and asked me what she did wrong, i can't seem to find the problem. does someone know what it is?</p>
  <p>the code she send me:</p>
  <pre><code># 10*. Pipe that to a ggplot command and create a histogram with 4 bins. 
  # Hint: you will NOT write ggplot(df, aes(...)) because the df is already piped in. 
  # Instead, just write: ggplot(aes(...)) etc.
  # Title the histogram, &quot;Distribution of Sunday tips for bills over $20&quot;
  # Feel free to style the plot (not required; this would be a typical exploratory
  # analysis where only you will see it, so it doesn't have to be perfect).
  df %&gt;%
    filter(total_bill &gt; 20 &amp; day == &quot;Sun&quot;) %&gt;%
    ggplot(aes(x=total_bill, fill=size)) +
      geom_histogram(bins=4) +
      ggtitle(&quot;Distribution of Sunday tips for bills over $20&quot;)
  </code></pre>
  <p>the error:</p>
  <pre><code>Error in df(.) : argument &quot;df1&quot; is missing, with no default
  </code></pre>