Month: June 2010

Sounds cool … looks uncool

I have been presenting “bad examples” of Sankey diagram before (like this one, or this one), and I have a few more in my collection.
The following one from an PowerPoint presentation by someone from PennState. The topic of the presentation is actually quite interesting: thermoacoustic refrigeration – using sound waves for cooling. It describes the results of a research project.

Unfortunately, this Sankey diagram has some shortfalls, which qualify it as being a “bad example”. These are:

  • Sankey arrow widths are not always to scale (see for example the flow of 7.6 W in the middle labelled “surface”, which is definitely not half the width of the 14.6 W flow marked “Joule loss”)
  • Arrows branching out of the main flow are shown as short stubs, just as if the quantity is represented by the length (!) of the arrow, rather than its width. A completely new concept for Sankey diagrams. Have a look at the 0.9 W mini-stub “Surface” in the cone area, and at the 1.2 W stub “Surface” in the linear motor box.
  • The diagram uses dotted-dashed frames to mark sections in the diagram, at the same time it uses white arrows with a simple border line for the Sankey arrows. Either colored or grey areas, or colored Sankey arrows would have helped enormously.
  • Arrows branch off orthogonally, and to accomadate for the reduced quantity in the remaining flow, the border of the arrow on the opposite side curves in. This is not wrong as such, but rather uncommon, and can best be observed in the linear motor section at the left. Same phenomenon for arrows joining the main arrow. The arrow heads just overlap rather than actually joining the other arrow graphically. The receiveing arrow bulges out on the other side.
  • The section between the exhaust heat exchanger and the regenerator is awkward, and probably wrong. How can you branch off 296.6 W and 5.8 from a stream of 320.5 W, and are still able to have a flow of 63.0 W lead away from the remaining flow. I have not yet figured out what the problem is, but possibly the “Carnot minimum” flow was supposed to be drawn the other way round.

When I get to figure out the last issue I might draw this Sankey diagram myself, and post it here.

Excelling in Excel

Does it take a hero like Daniel Ferry from the Excelhero blog to do a Sankey diagram in Excel? Well, you may not have to be a hero, but as Daniel puts it, this is “… seriously tedious work, as Excel has no native chart type to do what is required automatically.”

Daniel used the LLNL 2008 U.S. energy flow charts as model for his Sankey diagrams in Excel. Here is what he came up with (clipped section):

Actually the result is quite close to the original Sankey diagrams, with similar colours, arrow routing, and even the same fonts.

Daniel explains:

“I lightened the colors on the input boxes (had to do it), but otherwise I think my rendition is faithful to the original. I may have stayed too true to flow pipe proportionality. Some of them are so thin they do not print well. This should be addressed. While my pipes are seemingly lined up, they will not survive the chart being resized vertically without some small errors, either gaps in a pipe stack, or overlap.
An interesting project would be to create an Excel addin that would allow you to specify category box locations and have VBA do all of the grunt work in lining up the flow pipes, automatically creating the chart. (…) In it’s current form no VBA was used. “

So, if you wish to draw a Sankey diagram with the Microsoft Office package installed on your computer, and enjoy using VBA, you might want to give it a shot. Gabor Doka’s Sankey helper (an Excel macro) is another option. Dedicated Sankey diagram software tools are available. It would recommend one of those, if you need to produce more than one Sankey diagram, or wish to make updates to your diagram and layout adaptations more comfortably.

BTW, here is the story on the man behind the Sankey diagrams at LLNL.