I have set the y-axis range to (-10, 10) and then drawn two QCPItemStraightLines with their type set to ptViewportRatio and their y-values set to 0 and 1 respectively. When I tried to obtain their coordinate values in the coordinate space based on their pixel coordinates, I found that the y-values were not exactly -10 or 10, but floating-point numbers that were approximately 10. The difference is not a negligibly small number, so the error cannot be eliminated by setting the precision. What could be the reason for this?