Interactive where user can draw a line (marking on correct position)

Hi I'd been asked to set up some drag-drop where the student needed to take pre-made elements of a graph to drop into the correct place however because the elements are not square it meant the 'drop zone' was pretty much the whole graph, making it impossible for them to fail so we had to change our minds on this task.

It gave me an idea though - if it were possible to create a task where the user uses a 'line tool' like ms paint to draw a line (or more) on a background image or blank where the position of the line is judged for accuracy. Line tool being the one where it makes a straight line not the brush tool.

The class in question is a university level economics class where the lecturer wanted students to draw or drag over the line of equilibrium (ie a diagonal line) so the drag/drop didn't really work for that because the drop zone square for a diagonal line takes up the whole graph. 

Could be used to have students complete graphs, draw the line of best fit on a scattergraph, draw associations between objects, divide groups into subgroups, complete images (like for littler kids complete this triangle or something). 

Then to build it the teacher would add the background image (if needed) then use a line tool to draw where the lines need to go but then when the student sees it they only see the background image and have to add the lines in themselves. Then it can judge (with some grace amount) how accurate they are. 

Is this possible? Would anyone else find this useful?