I shouldn't see the "Add another series" button in the data tab of chart settings if I'm already using all columns

[mazameli]

As you can see from my underlying data in this line chart, I already have both of my metric columns assigned to the y-axis, but I'm still seeing the "add another series" link/button:

If I click it, I can pick from the already-being-used columns:

If I select one, it removes the other instance of it and leaves me with an empty dropdown: