title: How to fill a single 'pch' point on the plot with two-colours?
body: <p>If you take the code below, how can you change the filling of the third diamond, so it will be half-black, half-white? The solution should apply with any colors.</p>
  
  <pre><code>data &lt;- c(1,2,3)
  plot(data, pch=c(23,18,23), cex=c(2.5,3,2.5))
  </code></pre>