That was a huge surprise to me too. In some places ggplot2 seems to blur the lines between aesthetics and parameters.

This is from [geom_abline](http://docs.ggplot2.org/current/geom_abline.html)

```R
# Slopes and intercepts as data
p <- ggplot(mtcars, aes(x = wt, y=mpg), . ~ cyl) + geom_point()
df <- data.frame(a=rnorm(10, 25), b=rnorm(10, 0))
p + geom_abline(aes(intercept=a, slope=b), data=df)
```

`intercept` and `slope` are parameters to `stat_abline`.

It turns out you can map stuff to any parameters. So `aesthetic mappings` should just be thought of as mappings.