title: Why is bins parameter unknown for the stat_density2d function? (ggmap)
body: <p>I have been stuck with this for hours. When I run this :</p>
  
  <pre><code>library(ggmap)
  set.seed(1)
  n=100
  
  df &lt;- data.frame(x=rnorm(n, 0, 1), y=rnorm(n, 0, 1))
  
  TestData &lt;- ggplot (data = df) +
    stat_density2d(aes(x = x, y = y,fill = as.factor(..level..)),bins=4, geom = "polygon",) +
    geom_point(aes(x = x, y = y)) +
    scale_fill_manual(values = c("yellow","red","green","royalblue", "black"))
  </code></pre>
  
  <p>I get this error message :</p>
  
  <pre><code>Error: Unknown parameters: bins
  </code></pre>
  
  <p>Does anyone know why?</p>