Correct the Sankey plot : add benchmark option and adapt to the new toy data set

[lindadelacombaz]

Closes #106.
Dear @maurolepore,

Here is a quick correction of the Sankey plot, I have realized that the changes in the new toy data set also impacted this plot. I guess it is really a good time to create more tests :-). I have not the time to create the tests this week but will do it next sprint!

Also, somehow if I do not add the _arg suffix to name my variables in my function, my data does not filter correctly.
I have no idea why it is the case, for now I have kept it like that so Anne can use the package.

Thank you !

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[maurolepore]

LGTM. See my suggestions.