title: ggplot2::geom_tile can not assign fill color?
body: <p>I have a dataframe and I want to plot with <code>geom_tile</code> and fill according to the "ID" variable in the data, but I find that <code>fill</code> is not working in my code. Thanks for any help.</p>
  
  <pre><code>set.seed(1234)
  x &lt;- sample(x=-2288200:3076160, size = 1000,replace = F)
  y &lt;- sample(x=353334.1:4803914.0, size = 1000,replace = F)
  ID &lt;- sample(x=1:4, size = 1000,replace = T)
  Mydata &lt;- data.frame(x=x,y=y,ID=factor(ID))
  
  group.colors &lt;- c("1"="red","2"="yellow","3"="blue","4"="green")
  ggplot() + geom_tile(data = Mydata, aes(x = x, y = y, fill=ID)) + 
    scale_fill_manual(values = group.colors,name = "group")
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/MTZoX.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/MTZoX.png" alt="enter image description here"></a></p>