New Post: Movable (drag-able) Annotation

publicclass SelectableArrowAnnotation:ArrowAnnotation { // any better name?

        DataPoint lastPoint = DataPoint.Undefined;
        bool moveStartPoint = false;
        bool moveEndPoint = false;
        OxyColor originalColor = OxyColors.White;

        protectedoverridevoid OnMouseDown(object sender, OxyPlot.OxyMouseEventArgs e) {
            if (e.ChangedButton != OxyMouseButton.Left) {
                return;
            }
            
            lastPoint = this.InverseTransform(e.Position);
            moveStartPoint = e.HitTestResult.Index != 2;
            moveEndPoint = e.HitTestResult.Index != 1;
            originalColor = this.Color;
            this.Color = OxyColors.Red;
            var model = this.PlotModel;
            model.InvalidatePlot(false);
            e.Handled = true;
        }
        protectedoverridevoid OnMouseMove(object sender, OxyPlot.OxyMouseEventArgs e) {
            var thisPoint = this.InverseTransform(e.Position);
            double dx = thisPoint.X - lastPoint.X;
            double dy = thisPoint.Y - lastPoint.Y;
            if (moveStartPoint) {
                this.StartPoint = new DataPoint(this.StartPoint.X + dx, this.StartPoint.Y + dy);
            }

            if (moveEndPoint) {
                this.EndPoint = new DataPoint(this.EndPoint.X + dx, this.EndPoint.Y + dy);
            }

            lastPoint = thisPoint;
            var model = this.PlotModel;
            model.InvalidatePlot(false);
            e.Handled = true;
        }
        protectedoverridevoid OnMouseUp(object sender, OxyPlot.OxyMouseEventArgs e) {
            this.Color = originalColor;
        }
    }