I'm trying to understand how to have more than one series on a
plot, using the following data.
Year <- c('1950', '1960', '1970', '1980')
Bus <- c(10,20,30,40)
Bus.sd <- c(1.1, 2.2, 3.3, 4.4)
Car <- c(20, 20, 40, 40)
Car.sd <- c(1.1, 2.2, 3.3, 4.4)
sample_data = data.frame(Year, Bus, Bus.sd, Car, Car.sd)
qplot(Year, Bus,
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