Drawing lines on top of an image

[Larry Martell]

I am trying to make an 'image ruler.' Basically, I want to add the ability for the user to be able draw a line between 2 points on the image. They should be able to click and drag to create the line and once a line is drawn, move an endpoint around. I would need to be able to get the endpoints of the line in some units that would enable to be know the length of the line in pixels and also the x and y coordinates. I'd also want to show a box in the upper corner of the image that showed the length of the line in pixels. Can anything like this be done with smallbox? If not, does anyone know of a plugin that will allow me to do this?