Here is a toy version of my problem

```
x = runif(10);
y = runif(10);
z = (x+y)*(x-y);

```

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. **Please note I am not looking for an answer that generates z for these missing values**, 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 `filled.contour` 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 *per* *se*.