plotcov(C, mu, varargin)

Visualise a 2x2 covariance matrix by plotting ellipses at 1, 2 and 3 standard deviations.

Assuming `size(X,2) == 2` then
```
plotcov(cov(X), mean(X));
```

will draw ellipses at 1, 2 and 3 standard deviations.

See also (or just run) `plotcov_demo` for an example.