Don't override custom domain set on x/yScale option

I'm graphing data with sparse datapoints, but I want to fix the domain of the graph to match a given query, rather than discovering the domain based on the presence of data points.

For that, I had to use a custom `xScale`. The current code overrides the domain and range of the custom `xScale` though, so this PR changes the code to only override the domain if it wasn't set externally.

There might be a better way to do this, but that was the simplest I found. Feedback welcome!