Add plot legend in ggplot2

Question

How can I add a legend to my plot with this dataset? I would like to plot all variables in the same plot and identify the lines by names.

    Data         NO2         SO2             O3
    2004-01-01  24.49864    2.756818    30.17857
2   2004-01-02  33.40000    3.912609    22.45514
3   2004-01-03  27.55435    5.654783    24.33342
4   2004-01-04  24.87391    8.910000    30.64569
5   2004-01-05  41.96348    10.893478   44.92825
6   2004-01-06  48.80913    13.417391   44.91305
7   2004-01-07  49.10217    16.808696   28.34968
8   2004-01-08  49.14217    26.273913   16.61955
9   2004-01-09  34.52261    8.438261    17.80235
10  2004-01-10  45.33087    7.955217    36.34493

http://stackoverflow.com/questions/3777174/plotting-two-variables-as-lines-using-ggplot2-on-the-same-graph — user295691, Aug 14 '15 at 20:41
In short, you have to specify `color="NO2"` in the aesthetics for the `NO2` `geom_line`, and similarly for the others, or use `reshape2`/`tidyr` to melt the data first — user295691, Aug 14 '15 at 20:43
http://stackoverflow.com/questions/9531904/plot-multiple-columns-on-the-same-graph-in-r — Enrique Pérez Herrero, Aug 14 '15 at 21:00

score 1 · Answer 1 · answered Aug 14 '15 at 20:49

You can use gather from tidyr to reshape your data frame to the long format before plotting.

library(tidyr)
library(ggplot2)

# reshape to long format
datL <- gather(dat, Var, value, -Data)

# plot
ggplot(datL, aes(x = Data, y = value, colour = Var, group = Var)) +
  geom_line()

Here, dat is the name of your data frame.

score 0 · Answer 2 · edited May 23 '17 at 11:51

library(reshape2)
library(ggplot2)

s <-
"Date         NO2         SO2             O3
2004-01-01  24.49864    2.756818    30.17857
2004-01-02  33.40000    3.912609    22.45514
2004-01-03  27.55435    5.654783    24.33342
2004-01-04  24.87391    8.910000    30.64569
2004-01-05  41.96348    10.893478   44.92825
2004-01-06  48.80913    13.417391   44.91305
2004-01-07  49.10217    16.808696   28.34968
2004-01-08  49.14217    26.273913   16.61955
2004-01-09  34.52261    8.438261    17.80235
2004-01-10  45.33087    7.955217    36.34493
"

df <- read.delim(textConnection(s), sep="")
df$Date <- as.Date(df$Date, "%Y-%m-%d")
df <- melt(df, id.vars="Date")
ggplot(df, aes(Date, value, col=variable)) + 
    geom_line()

This question is duplicated from plot multiple columns on the same graph in R

Add plot legend in ggplot2

2 Answers2