I am using ggplot2's `scale_colour_gradient2` to have a color gradient scale for numbers from 1 to 20 using:

```
geom_line(aes(x=x, y=y, colour=c)) + 
  scale_colour_gradient2(lims=c(1, 20), high="red", low="grey")

```

The problem is that this puts value at 1 as white, and values at 20 as red, and I'd like to reverse this (smaller values in the `c` column for colour are more red than higher values. How can I do this? If I reverse `low` and `high` in `scale_colour_gradient2` it just gives a purpose gradient which is not what I intended. thanks.