A line control would typically consist of just two control points. My shape editor is not flexible enough for that unfortunately, all its shapes have four control points on a rectangular boundary. You could fake it by creating basically a rectangle shape but instead of drawing the rectangle you draw a line from the top left to the bottom right corner. That would work for slanted lines, probably gets you into trouble for perfectly horizontal or vertical lines though...

You'll have to generalize my shape editor code to be able to work with boundaries of any number of corners (2 for lines, 3 for triangles, 4 for rectangles / circled and more for polygons). That's probably not easy but st least you have a start :P