title: geom_smooth in ggplot2 not working/showing up
body: <p>I am trying to add a linear regression line to my graph, but when it's run, it's not showing up. The code below is simplified. There are usually multiple points on each day. The graph comes out fine other than that. </p>
  
  <pre><code>    b&lt;-data.frame(day=c('05/22','05/23','05/24','05/25','05/26','05/27','05/28','05/29','05/30','05/31','06/01','06/02','06/03','06/04','06/05','06/06','06/07','06/08','06/09','06/10','06/11','06/12','06/13','06/14','06/15','06/16','06/17','06/18','06/19','06/20','06/21','06/22','06/23','06/24','06/25'),
                    temp=c(10.1,8.7,11.4,11.4,11.6,10.7,9.6,11.0,10.0,10.7,9.5,10.3,8.4,9.0,10.3,11.3,12.7,14.5,12.5,13.2,16.5,19.1,14.6,14.0,15.3,13.0,10.1,8.4,4.6,4.3,4.7,2.7,1.6,1.8,1.9))
  
  
  gg2 &lt;- ggplot(b, aes(x=day, y=temp, color=temp)) +
    geom_point(stat='identity', position='identity', aes(colour=temp),size=3)
  
  
  gg2&lt;- gg2 + geom_smooth(method='lm') + scale_colour_gradient(low='yellow', high='#de2d26') 
  
  gg2 &lt;-gg2 + labs(title=filenames[s], x='Date', y='Temperture (Celsius)') + theme(axis.text.x=element_text(angle=-45, vjust=0.5))
  
  
  gg2
  </code></pre>
  
  <p>It's probably something really simple, but I can't seem to figure it out. Or it's the fact I am using a date for the x-axis, but I'm not receiving any errors. If it is due to the date, I'm not sure how to approach it. Thanks.</p>