title: how to plot the mean over time in ggplot2 R
body: <p>I'm looking to plot the mean of time on site over time. My dataset is called APRA, it has a column called Post_Day which contains the date as POSIXct and a column called Visit_Time_Per_Page_(Minutes) which is a num format.</p>
  
  <p>When I type this:</p>
  
  <pre><code>ggplot(APRA,aes(Post_Day,mean(`Visit_Time_Per_Page_(Minutes)`)))+
    geom_line()+
    labs(title = "Time on Page over Time", x = "Date", y = "Time on Page (Minutes)")
  </code></pre>
  
  <p>I get this back:</p>
  
  <p><a href="https://i.stack.imgur.com/TAAYM.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/TAAYM.png" alt="enter image description here"></a></p>
  
  <p>What I'm after is the daily average plotted over time.</p>
  
  <p>Thanks.</p>
  
  <p>Sample of data:</p>
  
  <pre><code>Post_Title  Post_Day    Visit_Time_Per_Page_(Minutes)
  Title 1     2016-05-15  4.7
  Title 2     2016-05-15  3.8
  Title 3     2016-05-15  5.3
  Title 4     2016-05-16  2.9
  Title 5     2016-05-17  5.0
  Title 6     2017-05-17  4.3
  Title 7     2017-05-17  4.7
  Title 8     2017-05-17  3.0
  Title 9     2016-05-18  2.9
  Title 10    2016-05-18  4.0
  Title 11    2016-05-19  6.1
  Title 12    2016-05-19  4.7
  Title 13    2016-05-19  8.0
  Title 14    2016-05-19  3.3
  </code></pre>