The reason I think non-zero based line charts are acceptable when non-zero based bar charts aren’t is simple-a bar that is 2x as tall as the bar next to it is interpreted to be twice the value. However, for a line chart, the comparison between each segment of the line isn’t so natural. If it’s my goal to show a trend over time, and that trend starts at 1 million, I’ll be leaving an awful lot of white space on my line chart if I start at zero, and my trend may be very muted if all the time-series value are near 1MM.

Here’s a post from Edward Tufte, the Godfather of modern Info Viz, on the subject:

“In general, in a time-series, use a baseline that shows the data not the zero point. If the zero point reasonably occurs in plotting the data, fine. But don’t spend a lot of empty vertical space trying to reach down to the zero point at the cost of hiding what is going on in the data line itself. (The book, How to Lie With Statistics, is wrong on this point.)

For examples, all over the place, of absent zero points in time-series, take a look at any major scientific research publication. The scientists want to show their data, not zero.

The urge to contextualize the data is a good one, but context does not come from empty vertical space reaching down to zero, a number which does not even occur in a good many data sets. Instead, for context, show more data horizontally!”

– Edward Tufte, October 18, 2001

Alex, you’re absolutely right about not adjusting for inflation. I can’t wait to check our your insights on that.

Naomi, I didn’t know about that setting in Excel. Thanks for pointing it out. I won’t ask why you know about it. I’m sure you never made a 3D graph in your life ;>

Small increases from year to year may look like a positive improvement, whereas in real terms, the revenues (for example) are falling because of inflation. I discuss this a little here. One of my pet peeves I guess..