incorrect grouping in plotting longitudinal data with ggplot

Question

I was trying to plot my longitudinal dataset with ggplot. Here is the example data to illustrate my problem.

library(MASS)
library(ggplot2)
library(dplyr)

# create data    
dat.tx.a <- mvrnorm(n=250, mu=c(30, 20, 28), 
                        Sigma=matrix(c(25.0, 17.5, 12.3, 
                                       17.5, 25.0, 17.5, 
                                       12.3, 17.5, 25.0), nrow=3, byrow=TRUE))
    dat.tx.b <- mvrnorm(n=250, mu=c(30, 20, 22), 
                        Sigma=matrix(c(25.0, 17.5, 12.3, 
                                       17.5, 25.0, 17.5, 
                                       12.3, 17.5, 25.0), nrow=3, byrow=TRUE))
    dat <- data.frame(rbind(dat.tx.a, dat.tx.b))
    names(dat) = c("measure.1", "measure.2", "measure.3")
    dat <- data.frame(subject.id=factor(1:500), tx=rep(c("A", "B"), each=250), dat)
    rm(dat.tx.a, dat.tx.b)
    dat <- reshape(dat, varying=c("measure.1", "measure.2", "measure.3"), 
                   idvar="subject.id", direction="long")
#ggplot
dat %>%
        group_by(tx,time) %>%
        summarize(mean=mean(measure), sd=sd(measure)) %>% 
        ggplot(aes(x=time, y=mean, color=tx))+
        geom_point()+
        geom_line(aes(group=1))+
        labs(x='Time', y='Cytokine Level')

Here is the plot I got. The lines are totally screwed. What was wrong with the plot? thank you.

Answer 1

You need to replace group=1 by group=tx:

dat %>%
  group_by(tx,time) %>%
  summarize(mean=mean(measure), sd=sd(measure)) %>% 
  ggplot(aes(x=time, y=mean, color=tx))+
  geom_point()+
  geom_line(aes(group=tx))+
  labs(x='Time', y='Cytokine Level')