## Graphical equity 2

##### Oct 02, 2006

Based on my last post, Zuil and Lope engaged in a lively conversation about "flow charts", apparently also called "Sankey charts" in some circles.  Here is an example Zuil found at the EIA site:

Zuil commented that

Though often difficult to draw, Sankey diagrams are IMHO unbeatable to represent any type of lossless flow (energy, money, fluids, etc).

I mostly agree: flow charts are great at tracing flows, and it's easy to figure out proportional sources and uses from this example.  Moreover, as Lope suggested, it's fun (to read).

But... the data content of this chart is lower than that of the network graph or the Marimekko.  Imagine removing all the lines (arcs) in the network graph: that is what the flow chart includes.  It achieves more readability by simplification.

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Interesting discussion as usual, Kaiser.

You are correct that the diagram I pointed to encodes less information than the alternatives you gave.

This needs not be the case, however. Here is another example of a Sankey diagram that conveys the same level of information as the network and Marimekko charts:
http://eed.llnl.gov/flow/pdf/US_water2000.pdf

The simpler diagram I pointed to simply used size to convey the magnitude of a number. As you can see from the second, it is possible to use color to track a flow from source to destination.

Other related topics you might want to check out are the hammock plot (a generalisation of the parallel coordinate plot, aka bump chart), and flow map layout, an algorithm for laying out flow maps.

I've made a Excel panel dot plot of the energy data. You can see it here.

panel dot plot

I prefer dot plots to mosaic charts. Excel can be used to make panel charts (Tufte's small multiples, Cleveland's trells) with a little transformation and dummy axis.

I'm not sure how you can prefer dot plots to mosaic charts - it's like preferring a hammer to a screwdriver. They are both useful, but for different tasks.