title: How does one creates a `ggplot2` plot that is composed of stacked squares?
body: <p>I would like to use R to create a plot (preferably with <code>ggplo2</code>) that looks like this one:
  <img src="https://i.stack.imgur.com/CarXU.png" alt="plot illustration"></p>
  
  <p>In this plot, every bar is composed of various numbers of squares, when each square designates a given amount. </p>
  
  <p>The only way I could think of was to create stacked barchart (like the one depicted below) and putting horizontal lines above the plot. But this looks pretty awkward to me.</p>
  
  <p><strong>EDIT</strong> Commenters to this question and a pointer to <a href="https://stackoverflow.com/questions/13701485/r-graphs-creating-tuftes-horizontal-bar-lines">another SO question</a> suggest that placing horizontal lines IS the way to solve my problem. However, what happens if one want to use buildig blocks other than squares such as circles, small pictograms etc?</p>