title: How to plot logit and probit in ggplot2
body: <p>This is almost surely a newbish question/</p>
  
  <p>For the dataset below I have been trying to plot both the logit and the probit curves in ggplot2 without success. </p>
  
  <pre><code>Ft Temp TD
  
      1  66 0
      6  72 0
      11 70 1
      16 75 0
      21 75 1
      2   70 1
      7   73 0
      12 78 0
      17 70 0
      22 76 0
      3   69 0
      8   70 0
      13 67 0
      18 81 0
      23 58 1
      4   68 0
      9   57 1
      14 53 1
      19 76 0
      5   67 0
      10 63 1
      15 67 0
      20 79 0
  </code></pre>
  
  <p>The code I have been naively using is</p>
  
  <pre><code>    library(ggplot2)
      TD&lt;-mydata$TD
      Temp&lt;-mydata$Temp
      g&lt;-    qplot(Temp,TD)+geom_point()+stat_smooth(method="glm",family="binomial",formula=y~x,col="red")
      g1&lt;-g+labs(x="Temperature",y="Thermal Distress")
      g1
      g2&lt;-g1+stat_smooth(method="glm",family="binomial",link="probit",formula=y~x,add=T)
      g2
  </code></pre>
  
  <p>Could you please tell me how I could improve my code so as to plot these two curves on the same graph?</p>
  
  <p>Thank you</p>