How to create a line plot with groups in Base R without loops?

Question

I need to create a simple line plot with groups using the following data:

test = data.frame(x =  rep(1:3, each = 2),
                  group =  rep(c("Group 1","Group 2"),3),
                  groupcd= rep(c(1,2),3),
                  y=   c(22,8,11,4,7,5)
                  )

I can easily do it with GGPLOT:

library(ggplot2)
    #GGPLOT
    qplot(x=x, y=y, 
           data=test, 
           colour=group, 
           main="GGPLOT line plot with groups") +
      geom_line()

I can also do it with TRELLIS:

library(lattice)
xyplot(y~x,
       type="b",
       group=group,
       data=test,
       main="TRELLIS line plot with groups",
       auto.key =list(
         lines = TRUE)
       )

However, I am a bit reluctant to use GGPLOT or TRELLIS right now. I'd like to be able to create this graph with Base R. The only way I can get this plot to work in Base R is by using for loop:

# set up empty plot
plot(test$y ~test$x,  ylab="y", xlab="x", type="n", main="Base R line plot with groups")
colors<-c("red","blue")
#plot each group in the for loop
number_of_groups <- as.numeric(max(unique(test$groupcd))) #calculate number of groups
for (i in 1:number_of_groups) 
{
  temp <- subset(test,  groupcd==i )
  lines(temp$x, temp$y, col=colors[i])  
  points(temp$x, temp$y, col=colors[i])  
}
legend("top", legend=unique(test$group), text.col =colors  )

This approach seems quite convoluted. Is there an easier way to do it in base R? Is there a group option in base R plot function? Thank you so much.

Answer 1

Different approach using MATPLOT:

library(reshape)
test = data.frame(x =  rep(1:3, each = 2),
                  group =  rep(c("Group 1","Group 2"),3),
                  groupcd= rep(c(1,2),3),
                  y=   c(22,8,11,4,7,5)
                  )
colors<-c("red","blue")

#Transform  data to wide format 
test_transposed<-reshape(test, 
                         idvar='x',
                         drop="group",
                         timevar="groupcd", 
                         direction="wide")
colors<-c("red","blue")

#drop x column
test_transposed$x<-NULL

matplot(test_transposed, 
        type = "b",
        ylab="y",
        col=colors,
        main="MATPLOT with groups",
        pch = 1:2)

legend("top", 
       legend=unique(test$group), 
       lty=1:2, 
       col=colors,
       pch=1:2  )

Answer 2

What about something like this as a base to work from:

test = data.frame(x = rep(1:3, each = 2),
                  group = rep(c("Group 1", "Group 2"), 3),
                  group_cd = rep(c(1, 2), 3),
                  y = c(22, 8, 11, 4, 7, 5))

xvals <- split(test$x, test$group)
yvals <- split(test$y, test$group)

plot(1:max(unlist(xvals)), ylim = c(0, max(unlist(yvals))), type = "n")
# thanks to @BenBolker for refining this next key line
mapply(lines, xvals, yvals, col = c("red", "blue"), pch = 1:2, type = "o")

Result: