ggplot2 line chart gives "geom_path: Each group consist of only one observation. Do you need to adjust the group aesthetic?"?

Question

With this data frame ("df"): year pollution 1 1999 346.82000 2 2002 134.30882 3 2005 130.43038 4 2008 88.27546I try to create a line chart like this: plot5 <- ggplot(df, aes(year, pollution)) + geom_point() + geom_line() + labs(x = "Year", y = "Particulate matter emissions (tons)", title = "Motor vehicle emissions in Baltimore")The error I get is:> geom_path: Each group consist of only one observation. Do you need to> adjust the group aesthetic?The chart appears as a scatter plot even though I want a line chart. I tried to replace `geom_line()` with `geom_line(aes(group = year))` but that didn't work. In an answer I was told to convert year to a factor variable. I did and the problem persists. This is the output of `str(df)` and `dput(df)`: 'data.frame': 4 obs. of 2 variables: $ year : num 1 2 3 4 $ pollution: num [1:4(1d)] 346.8 134.3 130.4 88.3 ..- attr(*, "dimnames")=List of 1 .. ..$ : chr "1999" "2002" "2005" "2008" structure(list(year = c(1, 2, 3, 4), pollution = structure(c(346.82, 134.308821199349, 130.430379885892, 88.275457392443), .Dim = 4L, .Dimnames = list( c("1999", "2002", "2005", "2008")))), .Names = c("year", "pollution"), row.names = c(NA, -4L), class = "data.frame")

Answer 1

Add group=1 to your ggplot.

For instance:

ggplot(df, aes(x=xvar, y=yvar, group=1)

This tells ggplot that you only have one bit of information per group. This is important when you are dealing with calculated averages, as ggplot tries to do that for you.