title: How can I set individual column widths in geom_tile (in ggplot2)
body: <p>I was wondering whether someone could help me with this:
  I want to set the widths of individual columns in a ggplot2 geom_tile heatmap to distinct values.</p>
  <pre><code>library(ggplot2)
  library(data.table)
  
  ggplot(data = melt(as.matrix(mtcars))) + geom_tile(aes(x=Var2, y=Var1,fill = value))
  </code></pre>
  <p>I would like the first column (mpg) to have a width of, say, 5, the second one 2, etc.</p>
  <p>thank you</p>