A good rule when making graphs is to remove needless impediments. Every extra act of interpretation we ask of the reader is a chance for them to misunderstand, be baffled, or get frustrated and move on. There should be as little standing between the reader and the data as possible. One level of interpretation all readers have to grapple with is the humble axis; here are some guidelines.
If you can, put units right there on the axis, not on the axis label. In general, getting information out of the label and caption and putting it on the graph where people can see it a good idea.
(By the way, that’s a real degrees sign above; all fonts have one. See Robin Williams for tips on finding it and other special characters. Never try to fake it with a superscripted o!)
Rotate for readability
All the graphing software I know generates vertical labels on the y-axis by default, but these are really almost unreadable. It’s a good idea to make them horizontal wherever possible, moving them to the top if there’s no space to the left.
I’m also a fan of getting axes out of log form. Real units are what we’re used to reading, and forcing people to calculate antilogs in their head increases the risk they’ll misread your numbers. We’re pretty terrible at comparing logarithmic values as it is, so it’s almost deceptive to hide them behind a linear axis.
When you expand units to make them comprehensible, they do take up more room. (For example, see below. But who doesn’t understand what Ma means, I hear you ask? Well, your mother, administrators, your congressman, journalists, and the voting taxpayers or undergraduates who pay your salary perhaps.) One solution is to rotate them a little: -30° in this case. It’s also possible to make the axis smarter; why not show geological periods, for example?
Use sensible units
If you’re using non-decimal units for some reason don’t use a decimal scale, even if it seems more “scientific”. Why make needless work for the reader?
(The foot and inch marks used here, by the way, are the prime marks, not the typewriter quotes next to the semicolon key—see Robin Williams again.)
When we count days, we think in months and years, not base 10. If you turn the scale into a calendar, it no longer needs a silly axis label like “day of year”. Of course, always identify months with a word or roman numeral, because 01/05/06 can mean Jan 5 or May 1. We’re all used to reading calendars, so a detailed scale is fine—note that a sufficiently-detailed one doesn’t need an axis line.
You Don’t Have to Start with Zero
Honestly. In some cases, it’s just meaningless, as there is no “value zero” to graph, such as with the days to the right. And having the y-axis pass through 1 means a data point might get tangled up with the tick marks on the axis, so there’s no reason not to leave a small gap.
There are also cases where beginning at zero would add pointless empty space to the graph; consider how little trend we’d be able to see if the graph on the right’s y-axis went from 0 to 110. So the answer is to eliminate empty space from the axis as much as possible without being actively deceptive.
William Cleveland, in The Elements of Graphing Data, often allows the scale to continue below zero, to “avoid interference” between the perpendicular axis and any zero-value data; he uses a dotted reference line to stand in for the dropped axis. Unfortunately, in the following graph this implies that ozone could exist in a concentration of less than zero parts per billion! The space below the reference line is in fact a misleading and uninhabitable no-man’s land.
Leaving a gap so points don’t hit the axis is OK, but extending the scale implies data values also continue. These days, with a better color palette, data on the axis no longer has to be a problem. Giving data points a thin white stroke allows them to intersect lines (and each other) while remaining visible; a little better than the jittered circles Cleveland was forced to use. (In the above makeover, you can see I also added units to the axes, rescued the labels from that horrible ALL-CAPS computer font, and condensed the empty space.)
Comments and suggestions welcomed, as always.