Representing a higher-dimensional chart
I think there is 2 ways to do it.



Dimensionality reduction

Using technique such as PCA, you can represent your data in lower number of dimensions.

This is a good approach if you want to represent all your data in one graph.



Dimension removal

Another way to visualize is simply to visualize only a few dimensions at a time. For example you can visualize a/b, c/d, a/c in 3 different 2D graphs.

While much simpler, this approach does not allow you to have a single graph visualization.