abline by groups

Hello,
I have 2 variables - x and y, that belong to separate groups.
I want to plot all the x and y together, but show separate abline  for each
group. It can be done in ggplot2, but is there a simpler way to draw
ablines by group?
e.g.,

mydata<- data.frame(x = 1:20+rnorm(20, -3, 1), y = seq(1,20,by=1), group =
rep(letters[1:5],20))
plot(x,y,col=mydata$group)  # need to get separate ablines for each
color-code

1. The code does not run as is.

2. abline for more than two points is nonsense for your data, since the 
groups are not at all on one line.

3. So you may want separate regressions? Or a whole model of the data 
with different intercepts and same slope? Or in one model with different 
intercepts and different slopes?

Hence we cannot help and for now I assume this is a homework problem and 
you have not even repeated the question properly.

Uwe Ligges

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