Sunday, February 27, 2011

Musical Stacked Histograms

This graphic has gotten a lot of play in the past few weeks. It purports to show the collapse of the global music industry. The image is discussed on Junk Charts where attention is directed at the mystery of the decline in CD sales around 2001. An interesting image, but as Michael DeGusta of Business Insider shows not all is what it seems. He details the graphic's recent history and notes several problems: It's really only US sales figures and it's not adjusted for inflation, among other problems. He provides other images of music sales data and this correction of the chart in 2011 dollars:
Here we see a problem with interpreting such an image. One commenter on Junk Charts notes:

"We might be able to tell if the Digital Sales area was not hidden behind the CD Sales area. Hence the problem with area charts."

To which another responds with the correct answer:

"Digital sales isn't hidden behind the CD sales - its stacked on top. I know it looks like a range of mountains, but it isn't. I think. That's the perceptual problem with charts stacked like this."

Yes, it is not one curve sitting behind another, like a range of mountains. The histogram information is stacked on top of the other curves. Stacked histograms are often used when displaying time dependent data, like for following. But not always.
Here is an image of a "living histogram" that organizes men and women of different heights into frequency classes. The men's histogram is easily seen. The women (dressed in white) are added or stacked onto the men's histogram classes. It does give an indication of the population heights for both, but it's more difficult to view the women's histogram alone. The same holds for the graphic of music sales. But I want to see the time axis extended further into the past, showing sales of 45s, 78s, and then even Edison wax cylinders!

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