确保键的笛卡儿产品出现在熊猫表中。

I have a Pandas dataframe that has two key columns, and I want to ensure that the Cartesian product of those keys exist in the table (because I'll have to make a 2D plot containing all combinations). I'm having trouble coming up with a reasonably brief and idiomatic way to do this.I have a Pandas dataframe that has two key colu