@JDWarner, sure, but I can't imagine someone putting in something *other* than a number for "spacing"!

Additionally, the docstring is informative enough about what spacing needs to be, so this whole section would be dramatically simplified if *all* error conditions converged on a single, simple `ValueError`, "incorrect value for `spacing` argument".

Finally, a much more likely error for spacing, giving a single number (e.g. thinking that the ratio of z to x/y is sufficient info) will fail with an uninformative error when `len(spacing)` fails.