title: Setting zValue in PyQtGraph
body: <p>I have a PlotWidget in PyQtGraph on which I draw a LinearRegionItem and set its zValue to 1100 (something large to try and get it to be on top of other items). Then I draw on ImageItems, with no specified zValue and when I do that, the LinearRegionItem disappears. Is there some sort of implicit zValue associated with ImageItems? Or is there a better approach to setting a zValue for an item to ensure it's always on top? Thank you in advance for the help.</p>