How to get a single trendline with multiple data s

Question

I have multiple sources of data over three decades. The data is discontiguous and overlaps in multiple places. I would like to plot the points for each data source in a different color but then add a single trendline that uses all of the data sources. The included code has some sample data and two plot examples. The first call to ggplot, plots a single trendline for all of the data. the second ggplot call, plots each source distinctly in different colors with its own trendline.

    library(ggplot2)
    the.data <- read.table( header=TRUE, sep=",", 
    text="source,year,value
    S1,1976,56.98
    S1,1977,55.26
    S1,1978,68.83
    S1,1979,59.70
    S1,1980,57.58
    S1,1981,61.54
    S1,1982,48.65
    S1,1983,53.45
    S1,1984,45.95
    S1,1985,51.95
    S1,1986,51.85
    S1,1987,54.55
    S1,1988,51.61
    S1,1989,52.24
    S1,1990,49.28
    S1,1991,57.33
    S1,1992,51.28
    S1,1993,55.07
    S1,1994,50.88
    S2,1993,54.90
    S2,1994,51.20
    S2,1995,52.10
    S2,1996,51.40
    S3,2002,57.95
    S3,2003,47.95
    S3,2004,48.15
    S3,2005,37.80
    S3,2006,56.96
    S3,2007,48.91
    S3,2008,44.00
    S3,2009,45.35
    S3,2010,49.40
    S3,2011,51.19") 
    ggplot( the.data, aes( the.data$year, the.data$value ) ) + geom_point() + geom_smooth()
    #ggplot( the.data, aes( the.data$year, the.data$value, color=the.data$source ) ) + geom_point() + geom_smooth()

The second call displays the colored data points and I would like to add a single contiguous trendline representing all of the years.

Answer 1

Like this:

ggplot(the.data, aes( x = year, y = value ) ) + 
    geom_point(aes(colour = source)) + 
    geom_smooth(aes(group = 1))

A few notes:

• Don't map aesthetics to an isolated vector like the.data$year. (Until you really know what you're doing, and know when to break that rule.) Just use the column names.

• Map the aesthetics that you want in separate layers in their respective geom calls. In this case, I want the points colored differently, but for the smooth line, I want the data grouped all together (group = 1).