title: Visualize ..count.. in a line graph with ggplot2
body: <p>I have a data frame with tweets (containing a timecode, tweet-id, text, etc.) and want to visualise the amount of tweets per hour. It works fine with a bar graph:</p>
  
  <p><a href="https://i.stack.imgur.com/5o9UK.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/5o9UK.jpg" alt="Bar graph of tweets per hour"></a></p>
  
  <p>I use the following code to produce the bar graph (<code>created</code> stores the timecode of a tweet in POSIX format):</p>
  
  <pre><code>  ggplot(data=tweets_frame, aes(x=created)) + 
       geom_bar(aes(fill=..count..), binwidth=3600) + 
       scale_x_datetime("Time") + 
       scale_y_continuous("Tweets")
  </code></pre>
  
  <p>I want to produce the same graph, but as line graph instead of as bar graph.</p>
  
  <p>I tried to just replace <code>geom_bar</code> with <code>geom_line</code>:</p>
  
  <pre><code>  ggplot(data=tweets_frame, aes(x=created)) + 
       geom_line(aes(fill=..count..), binwidth=3600) + 
       scale_x_datetime("Time") + 
       scale_y_continuous("Tweets")
  </code></pre>
  
  <p>Which resulted in this error message:</p>
  
  <blockquote>
    <p>Error in eval(expr, envir, enclos) : object 'count' not found</p>
  </blockquote>
  
  <p>I cannot figure out how to specify the <code>..count..</code> in a line graph.</p>