title: How to highlight a point along a curve in ggplot
body: <p>I have the below code to plot a probit model comparing the chance of success based on a maximum temperature value. Seems to work well, I'm happy with the plot. But I'm hoping to highlight the point along the curve where the probability is 50%, and then draw a line down to the x-axis to determine (and show) this value as well. Also hoping to include confidence intervals for this estimate. Any help would be greatly appreciated!</p>
  
  <pre><code>data &lt;- data.frame(MaxTemp = c(53.2402, 59.01004,51.42602,41.53883,44.70763,53.90285,51.130318,54.5929,43.697559,49.772446,54.902222,52.720528,58.782608,47.680374,48.30313,56.10921,57.660324,46.387924,60.503147,53.803177,52.27771,58.58555,55.74136,49.04505,46.816269,52.58295,52.751373,56.209747,51.733894,51.424305,50.74564,47.046513,53.030407,56.68752,56.639351,53.526585,51.562313), 
                     Success=c(1,1,1,0,0,1,1,1,0,0,1,1,1,0,0,1,1,0,1,1,1,1,1,1,0,1,1,1,1,1,1,0,1,1,1,1,1))
  TempProbitModel &lt;- glm(Success ~ MaxTemp, data=data, family=binomial(link="logit"))                   
  temp.data &lt;- data.frame(MaxTemp = seq(40, 62, 0.5))
  predicted.data &lt;- as.data.frame(predict(TempProbitModel, newdata = temp.data, type="link", se=TRUE))
  new.data &lt;- cbind(temp.data, predicted.data)
  std &lt;- qnorm(0.95 / 2 + 0.5)
  new.data$ymin &lt;- TempProbitModel$family$linkinv(new.data$fit - std * new.data$se)
  new.data$ymax &lt;- TempProbitModel$family$linkinv(new.data$fit + std * new.data$se)
  new.data$fit &lt;- TempProbitModel$family$linkinv(new.data$fit)
  (TempProb &lt;- ggplot(data, aes(x=MaxTemp, y=Success)) + 
    geom_point() +  
    geom_ribbon(data=new.data, aes(y=fit, ymin=ymin, ymax=ymax), alpha=0.5) +  
    geom_line(data=new.data, aes(y=fit)) +
    labs(x="Peak Temperature", y="Probability of Success") )
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/y62y1.png" alt=""></p>