title: plot polynomial regression line with ggplot stat_smooth
body: <p>I'm trying to create a scatter plot with second degree polynomial regression line using ggplot:stat_smooth.
  Here are the codes:</p>
  
  <pre><code>df.car_spec_data &lt;- read.csv(url("http://www.sharpsightlabs.com/
  wp- content/uploads/2015/01/auto-snout_car-specifications_COMBINED.txt"))
  
  df.car_spec_data$year &lt;- as.character(df.car_spec_data$year)
  
  df.car_spec_data %&gt;% group_by(year) %&gt;%
  summarise(maxspeed=max(top_speed_mph,
  na.rm=T)) %&gt;%  ggplot(aes(x=year, y=maxspeed,
  group=1))+geom_point(color='red',   alpha=0.3,
  size=3)+stat_smooth(method='lm', y~poly(x,2))
  </code></pre>
  
  <p>I got the following error message:</p>
  
  <pre><code>Error: Mapping must be created by `aes()` or `aes_()`
  </code></pre>
  
  <p>Thanks a lot. </p>