h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The pink bars are the numbers, in millions, of tablets shipped, quarter by quarter. The line shows the percentage of them that have voice capabilities. It started off at about 16% of two million units, dipped to about 11% of 1.9 million, then rose to about 25% of 3.5 million.

I understand what each of the components of that graph indicates, just not what they're supposed to explain when put together in the way they are. They forced the scale to look like some correlation when no significant correlation exists. The only pertinent information is that the percentage of tablets capable of calls has gone up. That sales in general have also gone up over the same period isn't all that important for the story, but by overlaying the two and adjusting their scales they've made it look like there is a significant correlation.

Using two different scales on the same axis is a common method of making misleading graphs. There's an example on this page. http://www.statisticshowto.com/misleading-graphs/
(Hint: Fox News does it all the time.)

It's the old thing about global warming and pirates.

http://bama.ua.edu/~sprentic/101%20Psych%20&%20Life--Correlation-causation.htm

It doesn't give a very clear representation of what it's trying to get across, that's for sure.

it's crudely trying to show, i think, the percentage and the actual number. I get that a lot on my economics course.