Group data and plot multiple lines

Question

I'd like to plot multiple lines in R for this dataset: (x = Year, y = Value)

School_ID   Year    Value
A           1998    5
B           1998    10
C           1999    15
A           2000    7
B           2005    15

Each school has data for different years. I'd like to have one line for each school.

Answer 1

Let's create some data:

dd = data.frame(School_ID = c("A", "B", "C", "A", "B"),
  Year = c(1998, 1998, 1999, 2000, 2005),
  Value = c(5, 10, 15, 7, 15))

Then to create a plot in base graphics, we create an initial plot of one group:

plot(dd$Year[dd$School_ID=="A"], dd$Value[dd$School_ID=="A"], type="b",
     xlim=range(dd$Year), ylim=range(dd$Value))

then iteratively add on the lines:

lines(dd$Year[dd$School_ID=="B"], dd$Value[dd$School_ID=="B"], col=2, type="b")
lines(dd$Year[dd$School_ID=="C"], dd$Value[dd$School_ID=="C"], col=3, type="b")

I've used type="b" to show the points and the lines.

Alternatively, using ggplot2:

require(ggplot2)
##The values Year, Value, School_ID are
##inherited by the geoms
ggplot(dd, aes(Year, Value,colour=School_ID)) + 
    geom_line() + 
    geom_point()

Answer 2

Is this what you want? You need group = School_id to tell ggplot2 to plot separate lines for each school. If you want the horizontal axis to incluude all years between 1998 and 2005, then remove factor in x = factor(year)

  library(ggplot2)

df = read.table(text = "School_id Year Value 
 A           1998    5
 B           1998    10
 C           1999    15
 A           2000    7
 B           2005    15", sep = "", header = TRUE)

ggplot(data = df, aes(x = factor(Year), y = Value, color = School_id)) +       
  geom_line(aes(group = School_id)) + geom_point()

Answer 3

The plot function in base R does not support grouping so you need to display your groups one by one. GGPLOT handles grouping well. I also suggest looking at Trellis XYPLOT which allows you to plot separate groups.

This is how you can create a basic grouped line plot using Trellis:

library(lattice)
rm(list = ls())     # clear objects  
graphics.off()      # close graphics windows   

test = data.frame(x =  rep(1:3, each = 2),
                  group =  rep(c("Group 1","Group 2"),3),
                  y=   c(22,8,11,4,7,5)
                 )
xyplot(y~x,
       type="b",
       group=group,
       data=test,
       auto.key =list(
         points = FALSE, 
         columns=2,
         lines = TRUE)
)