转自：http://stackoverflow.com/questions/3777174/plotting-two-variables-as-lines-using-ggplot2-on-the-same-graph

test_data <- data.frame(
var0 = 100 + c(0, cumsum(runif(49, -20, 20))),
var1 = 150 + c(0, cumsum(runif(49, -10, 10))),
date = seq.Date(as.Date("2002-01-01"), by="1 month", length.out=100))

How can I plot both time series var0 and var1 on the same graph, with date on the x-axis, using ggplot2? Bonus points if you make var0 and var1 different colours, and can include a legend!

A: The general approach is to convert the data to long format (using melt() from package reshape or reshape2)

require("reshape")
require("ggplot2")

test_data_long <- melt(test_data, id="date")  # convert to long format

ggplot(data=test_data_long,
       aes(x=date, y=value, colour=variable)) +
    geom_line()

http://i.stack.imgur.com/qkaZR.png