             temp_end_step_length = d2;
            }
            // red
            context.beginPath();
            context.setLineDash([4, 4]);
            var db1 = temp_end_step_length * length_of(a11, a12);
            context.moveTo(x_to_client(x_min), y_to_client((b1 - db1 - a11*(x_min))/a12));
            context.lineTo(x_to_client(x_max), y_to_client((b1 - db1 - a11*(x_max))/a12));
            context.moveTo(x_to_client(x_min), y_to_client((b1 + db1 - a11*(x_min))/a12));
            context.lineTo