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

with data frame ("df"):

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

i try create line chart 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 is:

geom_path: each group consist of 1 observation. need adjust group aesthetic?

the chart appears scatter plot though want line chart. tried replace geom_line() geom_line(aes(group = year)) didn't work.

in answer told convert year factor variable. did , problem persists. output of str(df) , 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") 

you have add group = 1 ggplot or geom_line aes().

for line graphs, data points must grouped knows points connect. in case, simple -- points should connected, group=1. when more variables used , multiple lines drawn, grouping lines done variable.

reference: cookbook r, chapter: graphs bar_and_line_graphs_(ggplot2), line graphs.

try this:

plot5 <- ggplot(df, aes(year, pollution, group = 1)) +          geom_point() +          geom_line() +          labs(x = "year", y = "particulate matter emissions (tons)",                title = "motor vehicle emissions in baltimore")