title: ggplot heatmap failing to fill tiles
body: <p>This (minimal, self-contained) example is broken:</p>
  
  <pre><code>require(ggplot2) 
  
  min_input = c(1, 1, 1, 2, 2, 2, 4, 4, 4)
  input_range = c(4, 470, 1003, 4, 470, 1003, 4, 470, 1003)
  density = c(
      1.875000e-01,
      5.598958e-04,
      0.000000e+00,
      1.250000e-02,
      3.841146e-04,
      0.000000e+00,
      1.250000e-02,
      1.855469e-04,
      0.000000e+00)       
  
  df = data.frame(min_input, input_range, density)
  
  pdf(file='problemspace.pdf')
  ggplot(df, aes(x=min_input, y=input_range, fill=density)) +
      geom_tile()
  dev.off()
  </code></pre>
  
  <p>Producing:</p>
  
  <p><img src="https://i.stack.imgur.com/gyTlQ.png" alt="gapped tiles"></p>
  
  <p>Why are there big gaps?</p>