Did you locate this great post - Decision trees, flow diagrams, sankeys in Tableau... here is a solution !!! by Olivier CATHERIN? He covers creating this sort of view and provides sample data and workbooks.
Chris Love expands on this with this post - https://www.theinformationlab.co.uk/2015/03/04/sankey-charts-in-tableau/
And Chris Martini takes it to another level in his blog post from last month - More options for your Tableau Sankey Diagram — DataBlick
I hope these give you some inspiration.
Let us know if you need additional assistance.