void ofxTLKeyframes::updateDragOffsets(ofVec2f screenpoint, long grabMillis){
	for(int k = 0; k < selectedKeyframes.size(); k++){
		//selectedKeyframes[k]->grabOffset = screenpoint - coordForKeyframePoint(selectedKeyframes[k]->position);
        selectedKeyframes[k]->grabTimeOffset  = grabMillis - selectedKeyframes[k]->time;
        selectedKeyframes[k]->grabValueOffset = screenpoint.y - valueToScreenY(selectedKeyframes[k]->value);
	}
}
ofVec2f ofxTLKeyframes::screenPositionForKeyframe(ofxTLKeyframe* keyframe){
    return ofVec2f(millisToScreenX(keyframe->time), 
                   valueToScreenY(keyframe->value));
}