title: ggplot2 colour geom_point by factor but geom_smooth based on all data
body: <p>In ggplot2, the following command <code>p &lt;- qplot(wt, mpg, data=mtcars, colour=factor(cyl))</code> taken from <a href="http://docs.ggplot2.org/current/geom_smooth.html">here</a> plots a scatter plot with each point coloured according to factor</p>
  
  <p>I would like to fit all data with a geom_smooth irrespective of factor but keeping the colour of individual points according to factor. <code>p + geom_smooth(method="lm")</code> does a linear fit on each factor. How do I do this?</p>