title: How to plot a contour/ heat map plot with 3 vectors?
body: <p>Here is a toy version of my problem</p>
  
  <pre><code>x = runif(10);
  y = runif(10);
  z = (x+y)*(x-y);
  </code></pre>
  
  <p>I would like to then generate a heatmap of z vs. (x+y) and (x-y). The problem is that z is a vector and is not defined over all combinations of y and x. <strong>Please note I am not looking for an answer that generates z for these missing values</strong>, that is not a possibility in the real version of the problem. This is just a minimal version to work with. All solutions I've been able to find, such as <code>filled.contour</code> need a matrix for z specified over a grid of the independent variables rather than just a set of (x,y,z) data points with no structure <em>per</em> <em>se</em>. </p>