Scatter plot - using colour to group points?

Another approach would be to use ggplot2.? 
Code can look a bit daunting to begin with but ggplot2 is a 
very versitile graphing package and well worth learning.

Simple example
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library(ggplot2)
mydata <- data.frame(site=c("A","A","A", "B","B","B"), time1 = 1:6, t1=c(23,24,13,7,19,12),
                        t2=c(7, 4,6,8,5,9))

p <- ggplot(mydata, aes(x=time1)) +
    geom_point(aes(y= t1, colour= site)) +
     geom_point(aes(y = t2, colour=site))

p   <- ggplot(mydata, aes(x=time1)) +
    geom_point(aes(y= t1, colour= site)) +
     geom_point(aes(y = t2, colour=site))

p <- p + scale_x_continuous('Time')+
              scale_y_continuous('Temperature')

p
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