This should work with recent desktop versions of Chrome or Firefox. It was coded using Vanilla JS.
The left half shows natural cubic splines, the right half shows the Hobby algorithm.
(Click on the links for videos about these topics.) Click
somewhere in the graph to add a point. You can also use the
mouse to move existing points. Press the Backspace key
to remove the last point and press
r to remove all points.
Press c to toggle between open and closed curves.
Press h to
toggle between showing both graphs and hiding the right
half or press n to
hide/show the other half. Press a digit to add a point on the graph after the n-th point. Press - (the minus sign) to add a point before the first point. Press 0 to revoke the effect of typing a digit (or the minus sign).
Press w to switch between the "curl" values 0 and 1 for endpoints in open Hobby curves.