The warning
geom_smooth() using formula 'y ~ x'
is not an error. Since you did not supply a formula for the fit, geom_smooth assumed y ~ x, which is just a linear relationship between x and y. You can avoid this warning by using
geom_smooth(formula = y ~ x, method = "lm")
The axis behavior you describe and the error about discrete values suggests that non-numeric values, i.e. text, are being passed to a function. Can you post the code you use to set the axis limits? It would also be useful to see the output of
summary(RW)
Place lines containing only three back ticks, ```, just before and just after the output.
```
paste the output here
```