[R] Basic question: why does a scatter plot of a variable against itself works like this?
Marc Schwartz
marc_schwartz at me.com
Wed Nov 6 17:52:22 CET 2013
On Nov 6, 2013, at 10:40 AM, Tal Galili <tal.galili at gmail.com> wrote:
> Hello all,
>
> I just noticed the following behavior of plot:
> x <- c(1,2,9)
> plot(x ~ x) # this is just like doing:
> plot(x)
> # when maybe we would like it to give this:
> plot(x ~ c(x))
> # the same as:
> plot(x ~ I(x))
>
> I was wondering if there is some reason for this behavior.
>
>
> Thanks,
> Tal
Hi Tal,
In your example:
plot(x ~ x)
the formula method of plot() is called, which essentially does the following internally:
> model.frame(x ~ x)
x
1 1
2 2
3 9
Note that there is only a single column in the result. Thus, the plot is based upon 'y' = c(1, 2, 9), while 'x' = 1:3, which is NOT the row names for the resultant data frame, but the indices of the vector elements in the 'x' column.
This is just like:
plot(c(1, 2, 9))
On the other hand:
> model.frame(x ~ c(x))
x c(x)
1 1 1
2 2 2
3 9 9
> model.frame(x ~ I(x))
x I(x)
1 1 1
2 2 2
3 9 9
In both of the above cases, you get two columns of data back, thus the result is essentially:
plot(c(1, 2, 9), c(1, 2, 9))
Regards,
Marc Schwartz
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