title: Draw a heatmap with "super big" matrix
body: <p>I want to draw a heatmap.<br>
  I have 100k*100k square matrix (50Gb(csv), numbers on right-top side and other filled by 0).</p>
  
  <p>I want to ask "How can I draw a heatmap with R?" with this huge dataset.<br>
  I'm trying to this code on large RAM machine. </p>
  
  <pre><code>d = read.table("data.csv", sep=",")
  d = as.matrix(d + t(d))
  heatmap(d)
  </code></pre>
  
  <p>I tried some libraries like heatmap.2(in gplots) or something.<br>
  But they are take so much time and memories.</p>