We are so busy creating sexy charts to illustrate some random data that we often forget to check if our chart really answers the question. Heck, most of the times we don’t even have one. Chart first, ask questions later.
One of the major differences between tables and charts is this: a tables says “here is your data, now go find the answers (they must be here, somewhere)”, while a good chart says “here is your answer”.
The more precise and clear our question is, the easier is to select the right data and the right chart. Let me give you a recent example. Robert Kosara, at EagerEyes, discusses the “swing states”. Several readers contributed with great alternative displays but, as I commented, there is a fundamental issue: if we want to see “the swing” that’s what should be displayed, not the election outcomes. This:
is different from this:
In the first chart, we can see that the Republican candidate won in Alaska in 1968. In the second chart, we know that, in 1968, in Alaska, there was a different outcome – a “swing”.
Sure we can infer the swing from the first chart, and our answer about the “swing states” is there somewhere, but only the second chart can provide a clear and concise answer.
It is prudent to keep all the data (who knows what the future will bring, right?), but we should always be aware of our loss aversion tendency, and make sure that our chart is displaying what it was designed for. Edit your chart without mercy, and let redundant or plainly useless data go. That’s the only way to highlight the patterns we are looking for.
Now, you may want to reevaluate your question to allow for a broader answer. That’s ok, but do it carefully. Add detail without breaking the pattern. For instance, we may want to know more about the direction of the swing:
Bottom line, make sure that what your chart says is aligned with what you asked. If you can use your question in the chart title that’s a good sign that you are on the right track.