Scatter plot opacity dataset selection fails for non-child datasets #244

When an opacity dataset is selected from the scatter plot toolbar's dataset flyout, the selected dataset is not added to the Point Plot action if it is not a child of the currently loaded point dataset.

As a result, the selection appears to be ignored, even though the dataset can be selected from the flyout.

Root cause: The Point Plot action only accepts datasets that are children of the loaded point dataset, while the dataset flyout allows selecting datasets outside that hierarchy.