Overlay line plots in ggplot2 - r

Question

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.
Here is the code I used to plot the figure above:
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")
Question: 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!
Edit: Dataframe df26 is provided below (as requested by a commenter):
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

Answer 1

You need to calculate the means. Then you can plot them.
Using dplyr
library(dplyr)
df26means <- df26 %>%
group_by(Month) %>%
summarize(C1 = mean(C1, na.rm = T))
Then add it to your plot:
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") +
geom_line(data = df26means, aes(group = 1), size = 1.25, color = "black")
I'd recommend using annotate to add a nice piece of text on the plot identifying that line as the mean line. To get it in the legend, you'd probably need to set df26means$Year = "Mean", convert df26$Year to a character, rbind the two dataframes together, then convert Year to a factor. The plot code would be simpler, but the data wrangling is a bit more complicated.