(Unless someone has a better answer ...)

I will be answering my own post :

Waterfall charts are very picky :
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- They do not accept any value linking to other cells in the workbook (eg. Title, Axis name, etc.)
- They do not accept "Combo Charts" which is what I need.
- With a big project, they can create an Excel meltdown where the calculation time is multiplied by 30, 50 or even 100 times what it should be (in rare cases where I have not observed any pattern, but going from 20 seconds to 15 minutes for the same task is not normal ... and with a restart of Excel or a Reboot, everything is going fine ...)

Logically speaking, stay away from these charts if you can