An interesting aspect of this distortion, and one that many designers might not think about, is how we deal with interpreting curved lines that are tracking along similar paths - for example, sales versus budget. Have a look at such a chart:
Now, if I was to ask someone where these lines diverge the most, the average person will suggest that the largest gap is in and around the apogee of the curve. But it's not correct! Would you believe me if I told you that the largest divergence is at the start and that the difference at the apogee is actually half way between the maximum and minimum? Here is the data:
The problem is that our visual system is looking at the minimal distance between the lines, we aren't looking at the vertical difference between the points that define the lines. Lines in a line chart are actually there to give some shape to show us a trend, they aren't the data!
The line chart below may be a better representation:
Here there is still the temptation to look at the distance between the lines but at least we have the points to help us. We could even add some further clarity using a combo chart:
It may not be ideal, but using the gestalt principles of enclosure will at least direct our visual system to the connected dots and give us some better understanding of the differences.
You might even get away with putting the curves back in!
Stephen Redmond is author of Mastering QlikView, QlikView Server and Publisher and the QlikView for Developer's Cookbook
He is CTO of CapricornVentis a Qlik Elite Partner.