Start up R in a fresh session and please use bellow code:
library(ggplot2)
df <- 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")
df[] <- lapply(df, as.numeric) # make all columns numeric
ggplot(df, aes(year, pollution)) +
geom_point() +
geom_line() +
labs(x = "Year",
y = "Particulate matter emissions (tons)",
title = "Motor vehicle emissions in Baltimore")
This will solve your problem