What approach for a simple GUI to edit and add points to a line

While the game is running. Imagine I have two parallel lines… I want to be able add a point to one of those lines, have it add a little handle to both lines at that point. Then the little handle can be clicked and dragged to warp the line in the direction it has been dragged.

So if someone clicks in the middle of the line and adds a point, they should be able to click on the handle they have made and drag up to create a bell shaped like curve of the two lines. This will be in run time… not an editor.

Not looking for anyone to give me the exact code. Of course any sample code would be very welcome. Mostly I would like if someone experienced out there would recommend an approach to creating this functionality.

All the best.

Sounds like this would be useful.

–Eric