Assigning many line colors based on group in ggplot

Question

Suppose I have some code like the following, generating a lineplot with a considerable number of lines (example taken from here)

library(ggplot2)
library(reshape2)

n = 1000
set.seed(123)

mat = matrix(rnorm(n^2), ncol=n)
cmat = apply(mat, 2, cumsum)
cmat = t(cmat)
rownames(cmat) = paste("trial", seq(n), sep="")
colnames(cmat) = paste("time", seq(n), sep="")

dat = as.data.frame(cmat)
dat$trial = rownames(dat)
mdat = melt(dat, id.vars="trial")
mdat$time = as.numeric(gsub("time", "", mdat$variable))


p = ggplot(mdat, aes(x=time, y=value, group=trial)) +
    theme_bw() +
    theme(panel.grid=element_blank()) +
    geom_line(size=0.2, alpha=0.1)

So here, "trial number" is my group producing all of these lines, and there are 1000 trials. Suppose I want to "group my grouping variable" now - that is, I want to see the exact same lines in this plot, but I want the first 500 trial lines to be one color and the next 500 trial lines to be another. How can I do this with ggplot? I've been poking around for some time and I can't figure out how to manually set the colors per group.

score 3 · Answer 1 · answered Feb 23 '20 at 05:36

Add a variable splitting the data into two groups, then add use it to color the lines in ggplot

dat = as.data.frame(cmat)
dat$trial = rownames(dat)
dat$group = rep(c("a","b"), each = n/2)
mdat = melt(dat, id.vars=c("trial", "group"))
mdat$time = as.numeric(gsub("time", "", mdat$variable))


p = ggplot(mdat, aes(x=time, y=value, group=trial, color = group)) +
  theme_bw() +
  theme(panel.grid=element_blank()) +
  geom_line(size=0.2, alpha=0.1)

score 2 · Accepted Answer · answered Feb 23 '20 at 05:30

2

One possible solution will be to create a new column with the index of the trial number and then using an ifelse condition, you can set different group based on the trial number and pass the grouping variable as color in aes such as:

mdat %>% mutate(Trial = as.numeric(sub("trial","",trial))) %>%
  mutate(Group = ifelse(Trial < 51,"A","B")) %>%
  ggplot(aes(x=time, y=value, group=trial, color = Group)) +
  theme_bw() +
  theme(panel.grid=element_blank()) +
  geom_line(size=0.2, alpha=0.8)

Is it what you are looking for ?

NB: I only use n = 100 to get smallest dataframe.

answered Feb 23 '20 at 05:30

dc37

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Oh.. there's a `color` in `aes` I've never used... geez. Thanks! – Eric Hansen Feb 23 '20 at 05:36
You're welcome ;). A lot of aesthetic parameters can be pass into `aes`. You will find a lot of example on SO. – dc37 Feb 23 '20 at 05:43

Assigning many line colors based on group in ggplot

2 Answers2