title: Use geom_smooth with transformed y
body: <p>Is there a way to use <code>geom_smooth</code> when the <code>y</code> variable in the formula is transformed? For example:</p>
  
  <pre><code>#This works:
   myplot &lt;- qplot(speed, dist, data=cars)
  (myplot + geom_smooth(method="lm", formula=y~log(x)))
  
  #does not work
  (myplot + geom_smooth(method="lm", formula=log(y)~x))
  </code></pre>
  
  <p>What I am after is a line like this:</p>
  
  <pre><code>myplot + geom_line(aes(x=speed, y=exp(predict(lm(log(dist)~speed)))))
  </code></pre>