You can select 3 points in an existing figure or a new one, and according to your selection - you can draw an ellipse.
The first 2 points are the vertices (the edges of the major / minor axis) and the 3rd point can be every point on the ellipse. In case that the user selects an impossible point (meaning no ellipse can pass through the 3 points), the function returns NaN's and the result in the figure window is just a flat line.
During the code development, I was assisted by Roger Stafford's answer and Wikipedia
http://www.mathworks.com/matlabcentral/answers/86615-how-to-plot-an-ellipse