title: Cannot get predicted probabilities from a logistic regression
body: <p>I made a GLM model with logit link in 2 ways. I want to know the chance to win according to these values, for example, for all combinations of these data. If I use <code>expand.grid</code>, I have the same chance to have <code>win=1</code> or <code>win=0</code> for the same combination of <code>height</code>, <code>weight</code> and <code>age</code>. But if I use <code>seq</code>, I don''t have all the combinations. </p>
  
  <p>I also want to be able to predict a probability to win with the following example data: <code>height=1.89</code>, <code>weight=90</code>, <code>age=18</code>. How can I do it?</p>
  
  <pre><code>set.seed(1)
  mydat &lt;- data.frame(
    win    = as.factor(sample(c(0, 1), 50, replace=TRUE)), 
    weight = runif(50, min=80, max=100),
    height = runif(50, min=1.75, max=1.9),
    age    = runif(50, min=18, max=30)
  )
  mod1 &lt;- glm(win ~ weight + height + age, data=mydat,       
              family=binomial(link="logit"))
  
  all.x &lt;- expand.grid(height=unique(mydat$height), weight=unique(mydat$weight), 
                       age=unique(mydat$age),win=unique(mydat$win))
  
  
  plotdat &lt;- data.frame(weight=seq(from=80, to=100, length.out=200),
                        height=seq(from=1.75, to=1.9, length.out=200),
                        age=seq(from=18, to=30, length.out=200))
  
  preddat1 &lt;- predict(mod1, newdata=plotdat, se.fit=TRUE)
  preddat2 &lt;- predict(mod1, newdata=all.x, type="response")
  
  dat1 &lt;- data.frame(fit=preddat1$fit,prob=exp(preddat1$fit)/(1+exp(preddat1$fit)), 
              weight=plotdat$weight, height=plotdat$height,
              age=plotdat$age)
  dat1 &lt;- dat1[order(dat1[,2]),]
  
  dat2 &lt;- cbind(all.x,preddat2)
  dat2 &lt;- dat2[order(dat2[,5]),]
  </code></pre>