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Thanks to my manager, we have found a workaround to this question.
1. Instead of keeping the original data source, he could use the pivot function to obtain this table:
This pivot function is surprisingly absent in my version of Tableau (though newer than his version) 2018.2.0.
2 Calculated fields added. Once the data are presented like this, it allows to obtain such graph, with a simple Parameter selection :
A second solution from Tableau support, also based on a new organization of data source, is to do a Pivot table first ("fournisseur" = "supplier"):
and then graph from it:
I hope that could help people with the same question, even if key words for this search makes it difficult to find online...