title: error on ggplot2 stat_smooth
body: <p>I receive an error when I try to use ggplot2 but I do not understand why.
  I am probably doing some little mistake somewhere... </p>
  
  <pre><code>    &gt; head(d)
        TargetGroup2012 TargetGroup2000     bmi3 age3 PA_Score education3 asthma3 allasthma3 tres3
      1               2               2 20.89796   55        2          2       0          0     0
      2               2               2 20.20038   49        3          2       0          0     0
      3               2               2 30.47797   58        3          1       0          0     0
      4               2               2 34.13111   51        2          2       0          0     0
      5               3               2 23.24380   52        3          1       0          0     0
      6               3               2 16.76574   62        2          3       0          0     0
        wheeze3 SmokingGroup_Kai groupchange
      1       0                4           4
      2       1                4           4
      3       0                5           4
      4       1                4           4
      5       0                3           5
      6       0                3           5
      &gt; 
  
      ggplot() + 
        stat_smooth(data=d,aes(x=bmi3,y=asthma3),
                    method="gam",
                    formula=asthma3~bmi3, 
                    family="binomial")
  
  # this is the ouput
  # Error in eval(expr, envir, enclos) : object 'asthma3' not found
  
  
  #this one works
  
      ggplot()+ stat_smooth(data=d,aes(x=bmi3,y=asthma3),
                    formula=asthma3~bmi3, 
                    family="binomial")
  </code></pre>
  
  <p>What's wrong with my code?</p>