axis grid lines "on top" option moves grid lines above key - not useful

[And0k]

not possible to put "key" above grid lines and "xy" fill behind
transparency helps for me now, but it is not exactly what i want

[jeremysanders]

This is a bit tricky to fix. If we forced all keys to be on top of everything, this might work, however some plots may rely on keys being under other plot items, so would break backward compatibility.