Thank you Michael.
Should have enough now to finish a catenary curve script hanging from two points in 3D, even with different z values, with
gravity in the -y direction. There is plenty of code on the internet.
Yes, the new x-axis "rotates" about the new y-axis, to "zero out" the z values.

Just figured out that a minus sign "-" is not the same as a dash "–".

So cut and paste of formulas with dashes for minus signs, from Microsoft Word to Notepad++, creates parse errors in javascript.