LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
Sankey Diagram (csaladen) - Sankey Diagram Generator
draw.io - Online diagramming application
Sankey Diagram - Professional drawing tool to create Sankey Diagrams.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
SankeyMATIC - There are no tools to install. There is no code to write. You do not have to sign up for anything. Just enter your data, customize the diagram to your liking, and download a finished product.