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

With this data frame (“df”):

year pollution
1 1999 346.82000
2 2002 134.30882
3 2005 130.43038
4 2008  88.27546

I 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")