In the online help:
For investment based graph types you can hold down the Shift key on your keyboard while making a selection from the dropdown combination box. Holding down the Shift key will cause the date range selection to be applied to all open investments. Otherwise, it is only applied to the topmost investment.
It seems the date range selection is only applied to the investments in the current sub-portfolio. I think I would prefer the behavior as described in the online help. What do you think?