mtcars ggplot doesn't know how to deal with class numeric

Question

I am just trying to make a simple plot using mtcars and ggplot:

ggplot(data=mtcars, aes(x=mpg,y=hp))+geom_line(mpg,hp,col=cyl)

but I get the er

Answer 1

My suggestion is to specify all aesthetics in one place and to coerce cyl to factor for colour coding as categorial variable.

ggplot(data=mtcars, aes(x=mpg, y=hp, colour=factor(cyl))) + geom_line()

will create

Answer 2

See if you get better results passing hte argument to aes inside geom_line:

ggplot(data=mtcars, aes(x=mpg,y=hp))+geom_line(aes(x=mpg,y=hp,colour=cyl) )

I think the results are a bit clearer if you add a grouping parameter to the basic aes spec:

ggplot(data=mtcars, aes(x=mpg,y=hp, group=cyl)) + 
        geom_line(aes(x=mpg,y=hp,colour=cyl) )

And even better legend anotation with a factor() on cyl:

ggplot(data=mtcars, aes(x=mpg,y=hp, group=cyl))+ 
       geom_line(aes(x=mpg,y=hp,col=factor(cyl)) )