QXYSeries.appendNp requires numpy<2 even for python>3.9, which is compatible with numpy>=2. As of today, the behavior is not documented at all. Please add the info into the docs and, if feasible and reasonable, add numpy>=2 support.

The following code reproduces the issue:

import numpy as np
from PySide6.QtCharts import QLineSeries

QLineSeries().appendNp(np.empty(1), np.empty(1))