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()