How to map additional data to Vega lite tooltip?

[danny-ha-genesys]

I'm trying to map an additional value (z) to the tooltip. How can I achieve this with the following schema I have so far? I know it should be along the lines of adding additional objects to the tooltip array (one for last month and one for this month), but I'm unsure on how to map the values correctly. I can't seem to figure it out from the documentations and not seeing any examples of this. Any help would be appreciated. Thanks.

editor