It is a free plug-in made with RhinoScript for Windows Rhinoceros 5 and 6. Command is: 5PtsEllipse.
This plug-in allows you to draw an ellipse using 5 points that must meet the following 3 conditions:
1) None is repeated (5 different points are needed to define an ellipse).
2) All are coplanar (the ellipse is a flat curve).
3) No 3 or more are collinear (3 or more collinear points define a line, not an ellipse).
Anyway, you can select 5 correct points and see a message that says "Invalid points (points do not define an ellipse)". It means that the 5 points define another conic (parabola or hyperbola).
Plugin will not work if you try to draw a very very small or very very large ellipse. These problems are not due to plug-in, but to units and tolerances of Rhinoceros. Anyway, when a problem happens, a message is displayed.
This version 1 is a test version. So, i will be very grateful if anyone informs me about any problem or error. Any suggestions are welcome too. You can add a review here or send me comments (firstname.lastname@example.org).