ofxSplineTool is an addon for openFrameworks
ofxSplineTool provides an interface for controlling a cubic spline. It's a hard fork of ofxCurvesTool, which emulates the "Curves" dialog in Photoshop.
The interface only listens to mouse and keyboard events when it is drawn. It's a little magical, but if you're interested in the details see the implementation for ofxSplineTool::drawEvent().
ofxSplineTool stores data in an anonymous list of lists, using a severely simplified YAML representation. Call ofxSplineTool::save() or ofxSplineTool::load() with a .yml extension.
If you happen to scale or rotate ofxSplineTool, it may get confused about where your mouse is clicking, but it's ok to translate it however you like.