It is difficult to make the above condition in a selection box, because you have non-exclusive grouping.
In alternative you can have (either in the calculated dimension or in the script):
Qlikview will perform better and allow you to select multiple values easily, to combine fruit and vegetables.
If you really need to have that unconventional group you can create buttons that make selections.In the button (label it 'fruit and veg') and set the action to select value 'Y' on the filed flood_flag and another action to select value 'N'.
I hope this helps.