Multiple Regression lines in R

Question

I am trying to have output 2 different graphs with a regression line. I am using the mtcars data set which I believe you can load into R. So, I am comparing 2 different pairs of information to create a regression line. And the problem seems to be that the 2nd regression line from the 2nd graph is for some reason in the first graph as well.

I just want it to show 1 regression line in each graph the way it should be.

mtcars
names(mtcars)
attach(mtcars)    

par(mfrow=c(1,2), bg="white")
with(mtcars,
{

regrline=(lm(gear~mpg))
abline(regrline)
plot(mpg,gear,abline(regrline, col="red"),main="MPG vs Gear")

# The black line in the first graph is the regression line(blue) from the second graph

regrline=(lm(cyl~disp))
abline(regrline)
plot(disp,cyl,abline(regrline, col="blue"),main="Displacement vs Number of Cylinder")

})

Also when I run the code separately for plotting, I don't see the black line. Its only when I run it with the: with() it causes a problem.

Answer 1

First of all, you really should avoid using attach . And for functions that have data= parameters (like plot and lm ), its usually wiser to use that parameter rather than with() .

Also, abline() is a function that should be called after plot() . Putting it is a parameter to plot() doesn't really make any sense.

Here's a better arrangement of your code

par(mfrow=c(1,2), bg="white")

regrline=lm(gear~mpg, mtcars)
plot(gear~mpg,mtcars,main="MPG vs Gear")
abline(regrline, col="red")

regrline=lm(cyl~disp, mtcars)
plot(cyl~disp,mtcars,main="Displacement vs Number of Cylinder")
abline(regrline, col="blue")

You got that second regression line because you were calling abline() before plot() for the second regression, do the line drew on the first plot.

Answer 2

Here is your code cleaned up a little. You were making redundant calls to abline that was drawing the extra lines.

By the way, you don't need to use attach when you use with . with is basically a temporary attach .

par(mfrow=c(1,2), bg="white")
with(mtcars,
{
    regrline=(lm(gear~mpg))
    plot(mpg,gear,main="MPG vs Gear")
    abline(regrline, col="red")

    regrline=(lm(cyl~disp))
    plot(disp,cyl,main="Displacement vs Number of Cylinder")
    abline(regrline, col="blue")
}
)