Question

I want to have multiple “lines” on the same plot. Multiple data points.

In my example, how can I include the ‘xa’ and ‘xb’ data points.

This is what I have for just one set of data points but I want two. How can I modify this script.

library(ggplot2)

da <- c("2012-02-02 09:01:00", "2012-02-02 09:02:00", "2012-02-02 09:03:00")
db <- c(0.4, 0.6, 0.5)

xa <- c("2012-02-02 09:01:00", "2012-02-02 09:02:00", "2012-02-02 09:03:00")
xb <- c(0.3, 0.43, 0.7)

da2 <- as.POSIXct(da)
dfx <- data.frame(da2, db)

summary(dfx)

png('time_data_errs6b.png', width=640, height=480)
gg <- qplot(da2, db, colour='red')+
    opts(title = 'Requests App')+xlab('Time')+ylab('Requests') +
    geom_line()

print(gg)
dev.off()

Answer 1

I would make two data.frames and merge them by the time variable.

> df.a<-data.frame(time=da, value.a=db)
> df.b<-data.frame(time=xa, value.b=xb)
> df.mix<-merge(df.a, df.b, by='time')

> df.mix
                 time value.a value.b
1 2012-02-02 09:01:00     0.4    0.30
2 2012-02-02 09:02:00     0.6    0.43
3 2012-02-02 09:03:00     0.5    0.70

Convert to POSIXct like you did then melt it to a long format.

> df.mix$time<-as.POSIXct(df.mix$time)
> df.melt<-melt(df.mix, id.vars='time')

ggplot deals with long format data very well, so its usually my goal to get data into a suitable structure with melt and merge before plotting.

> ggplot(df.melt, aes(x=time, y=value, colour=variable)) + geom_path()

I also like to uses the base ggplot rather than qplot for my own readability. but thats a matter of preference.