(x_to_client(x_max), y_to_client((b1 + db1 - a11*(x_max))/a12));
            context.strokeStyle="#ff8866";
            context.stroke();
            context.setLineDash([]);
            context.closePath();
            // blue
            context.beginPath();
            context.setLineDash([4, 4]);
            var db2 = temp_end_step_length * length_of(a21, a22);
            context.moveTo(x_to_client(x_min), y_to_client((b2 - db2 - a21*(x_min))/a22));
            context.lineTo(x_to_client(x_max), y_to_c