R: two scatterplots on single graph using ggplot

Question

Please note I am beginner with R. I have merged two data frames with one common column with merge() method. I have obtained data frame like:

 x   y1   y2
 1   3          


        

Answer 1

You should melt your data into long format and then map the colour aesthetic to the variable column from the melted data.frame. Something like this:

dat <- data.frame(x = c(1,2,1,3), y1 = c(3,2,2,5), y2 = c(5,4,2,5))

dat.m <- melt(dat, id.vars = "x")

ggplot(dat.m, aes(x, value, colour = variable)) +
  geom_point() +
  scale_colour_manual(values = c("red", "blue"))

You can manually set the limits with xlim() and ylim() respectively. It's not clear what you're doing with alpha, so I'll leave that one up to you.