title: Overlay line plots in ggplot2
body: <p>I've created a multiple line graph using ggplot2, where each line represents a year that is plotted against month (click link below).  Volume is represented on the y-axis.</p>
  
  <p><img src="https://i.stack.imgur.com/th7g6.png" alt="http://imgur.com/3Rwdjyi"></p>
  
  <p>Here is the code I used to plot the figure above: </p>
  
  <pre><code>ggplot(data=df26, aes(x=Month, y=C1, group=Year, colour=factor(Year))) + 
      geom_line(size=.75) + geom_point() +    
      scale_x_discrete(limits=c("Jan","Feb","Mar","Apr","May","Jun","Jul",
          "Aug","Sep","Oct","Nov","Dec")) + 
      scale_y_continuous(labels=comma) + 
      scale_colour_manual(values=cPalette, name="Year") +    
      ylab("Volume")   
  </code></pre>
  
  <p><strong>Question:</strong> How do I also include another line to the plot that represents the mean volume within each month with the ability to modify the line thickness and color of that mean line?  So far, all of my attempts at producing the right code have been unsuccessful (most likely due to my relative newbie status using R).  Any help is much appreciated!</p>
  
  <p><strong>Edit:</strong> Dataframe df26 is provided below (as requested by a commenter):<br></p>
  
  <blockquote>
  <pre><code>Year  Month   C1
  2010  Jan NA
  2010  Feb NA
  2010  Mar NA
  2010  Apr NA
  2010  May NA
  2010  Jun NA
  2010  Jul NA
  2010  Aug 183.6516764
  2010  Sep 120.6303348
  2010  Oct 85.31007613
  2010  Nov 13.7347988
  2010  Dec 20.93950545
  2011  Jan 13.35780833
  2011  Feb 14.16910945
  2011  Mar 9.786319721
  2011  Apr 41.24848885
  2011  May 122.3014387
  2011  Jun 422.4012809
  2011  Jul 539.8569592
  2011  Aug 527.6301222
  2011  Sep 385.8199781
  2011  Oct 201.7846973
  2011  Nov 27.91934061
  2011  Dec 7.919004379
  2012  Jan 10.22724424
  2012  Feb 10.64391791
  2012  Mar 88.06585438
  2012  Apr 124.0320675
  2012  May 325.1399457
  2012  Jun 465.938168
  2012  Jul 567.2273488
  2012  Aug 459.769634
  2012  Sep 333.8636373
  2012  Oct 102.0607986
  2012  Nov 23.18822051
  2012  Dec 15.64841121
  2013  Jan 7.458238256
  2013  Feb 4.34972039
  2013  Mar 26.2019396
  2013  Apr 38.82781323
  2013  May 257.0920645
  2013  Jun 357.594195
  2013  Jul 383.2780483
  2013  Aug 456.469314
  2013  Sep 319.3616298
  2013  Oct NA
  2013  Nov NA
  2013  Dec 17.01748185
  </code></pre>
  </blockquote>