tooltip question

Hi,
I have this bar chart:

bar.jpg
where:
A) the bar measures a quantity (i.e. total articles, 135)
B) the line measures a price (i.e. total cost $1350).
I would like to change the tooltip by making sure that in case A (bar) I show "Qty: xxxx" and in the other case (line) I show "Cost: xxxxxxx ".

I guess I have to write js code in PostFetch, but how do I recognize the two cases?

Attached Images

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