r - Using geom_abline() and ggplot -

i beginner in ggplot2--it's been 4 days since have started experimenting it. so, apologize if question sounds basic. i'd appreciate guidance--i've been struggling issue hour.

i experimenting using geom_abline() below:

 p <- ggplot(mpg, aes(cty, hwy)) + geom_point()  p + geom_abline() + facet_wrap(~cyl) 

this works in can see reference line in 4 faceted graphs below:

later, using related dataset mtcars see happens geom_abline()

  p <- ggplot(mtcars, aes(wt, mpg)) + geom_point()      p + geom_abline() + facet_wrap(~cyl) 

however, when ran command, couldn't see geom_abline(). quite surprisingly, found similar version of above command in file, , says "geom_abline() out of range"

while know "out of range" means, how know whether in particular dataset, abline() out of range? can override forcing use specific slope , intercept, i'd consider bit of hacking--i.e. modifying code after seeing output. there way can know happens behind scenes geom_abline()

here's graph got without abline()s

i'd appreciate thoughts. confused.

if don't give arguments, geom_abline() uses default values, intercept = 0 , slope = 1.

ggplot(mpg, aes(cty, hwy)) + geom_point() + geom_abline() + lims(x = c(0,35), y = c(0,50)) ggplot(mtcars, aes(wt, mpg)) + geom_point() + geom_abline() + lims(x = c(0,6), y = c(0,35))