Like a growing number of organizations, the Urban Institute maintains a style guide for data graphics. This guide specifies almost a dozen elements that should appear on charts, like axis labels and source lines. After recent discussions about a check list to help the blog team review charts and get them ready for publication on Urban Wire, I realized the guide says a lot about how and what the chart creator should do but very little about why. And there’s nothing like understanding why something is being done to align a team around the hows and whats.
With that in mind, I’d like to turn the style guide inside out, starting with our guiding intentions and connecting them to the specified element of a chart.
There are 10 listed here, but some charts will communicate well with fewer elements, or more, or have the same intention captured differently.
The general idea remains: the humble chart is a highly conventionalized form of communication, and good ones take care in similar ways.
1. A message
Focus this with selected data.
A story isn’t everything that happened. Likewise, a good chart isn’t all the data that ever existed. It’s an honest — not cherry-picked — selection of data put in a frame to communicate a point.
To create the clearest message, consider why you are making the chart. Perhaps the data show a trend that would make a strong impression visually. Perhaps, as with a jobs numbers report, whatever the data are doing is worth discussing. Or perhaps writing about numbers became too clumsy.
Even in this last example, there was a reason to discuss this particular slice of data. That reason is the chart’s message, and a good understanding of it should guide your selection of a chart type and inform consequent decisions about what to highlight and focus on.
Tell the reader where you’re headed with a title.
Like most communication lengthier than a tweet, a chart has a title to focus the reader and help her decide if she’s interested in reading on.
As this list item’s name implies, there a few ways to think about the role of the title.
A very newsy way to write a title is to “say what you see” and describe what’s on the graph, like “Half of respondents don’t like peas” or “Peas fail to please”. These are sometimes paired with a secondary statement (set in less prominent type) that unpacks the title, sharing details for those who have bought in to reading the chart and want to learn more — “A survey by XYZ Polling Service found 51.6 percent of people either disliked or strongly disliked the taste of peas.”
Another way to summarize the same graph would be, “Share of survey respondents indicating dislike of peas.”
Both work, but one does more to draw out a message from the chart, which has been shown to make the chart as a whole more memorable.
Put the issue in perspective with axis lines.
You will rarely see a chart with just one data point on it. Charts show relationships, one thing to itself over time or different categories of things. These relationships, usually shown vertically and horizontally, are the grid under the plot.
The axis should be clearly labeled and the categories on it should be well thought out. Too many and the story is incomprehensible, too few and it’s hard to know if the individual values fit into a trend or break it. The axis can also carry broad labels to clue a reader in, such as “more unequal →” on a chart displaying a complex measure of access to jobs.
Bonus: Consider whether transforming the values in your dataset would facilitate the message. Sometimes using percent change or the value of each data point relative to some other number cuts down on mental gymnastics for the reader. Sorting axis categories most to least or least to most is also an easy win for chart clarity.
4. Credibility (and courtesy)
Do this with a source line.
Showing where data came from helps readers judge the validity of the information and provides a path for further exploration.
A good chart provides enough of a trail that the reader could find the original information on her own.
Do this with notes.
As much as we try, not everything that needs to be known about a chart goes on the chart itself. Sometimes, we still need something like footnotes. Here we can define terms, spell out acronyms, give margins of error (if not shown on the chart itself), explain what data might be missing from a series, or present other limitations, including how data were collected.
A good chart isn’t trying to obscure anything. Be sure to leave a help line for any confusion you think might arise.
This idea also has a lot of overlap with —
Give your readers confidence about what they’re looking at with labels and/or a legend.
Charts are about visual communication, but that doesn’t mean the visuals should be asked to do all the work. Text has an important role, nudging the viewer toward your intended meaning.
Anything on the chart that’s encoded should be explained: the meaning of colors, heights, widths, and what’s on the axis, to name a few.
Bonus: Placing a label close to the thing it references (direct labeling), mirroring the order of categories from the chart in the legend, or mimicking their appearance are all good ways to strengthen connection between a label and its object. Try to keep labels horizontal to make them easy to read (this might mean rotating a column chart to a horizontal orientation to fit long category names).
Do this with a credit/byline or organization’s logo and branding.
If a chart really was just data you would only need a source. But the choices you make in creating a chart give it a frame, so own it!
Visual elements (including wordmarks) allow your organization’s authority to shine through, and, like the source, allows a reader to evaluate and learn more about where it came from.
Bonus: Thorough branding (standard fonts, proportions, colors) will help retain identity even if the chart is copied into another context without the credit line or logo.
8. A pathway
In a crowded landscape, help the reader get to the point with visual hierarchy.
Make the chart easy to ready by directing the reader’s attention with size, color, and placement.
Text that’s critically important should be big and bold; helper text should be small and light.
Prevent background elements from competing with the main event by turning them down, e.g. mute grid lines and contextual categories with grays or subtle colors.
9. Your work
Let the reader in on the thinking behind the chart by using detail in the units.
The numbers on a chart are interesting because they measure a specific thing. Whether it’s straightforward like “population (in millions)”, a little more layered like “density (people per square mile)”, or a lot more layered like “potential users (people within 10 miles without a car)”, well written unit labels will help readers understand what they’re seeing.
10. Nothing at all
Finally, sometimes the best chart is no chart!
A data visualization can take place in the imagination, as with a comparison like “the airplane is as large as a football field” or the insight from some data might be easily summarized with a multiple (“twice as many adults 65 and older lost jobs as any other age category”).
As our style guide recommends: “If you find your explanatory sentences do a better job of distilling the information, you might want to consider going without a chart.”
I might be biased, but I think charts are a big deal. Along with headlines, images, and pull quotes, they’re one of the most visible items on the page. They also telegraph authority. That’s a lot of responsibility!
I hope that unpacking the “why” behind these conventional elements will help us build charts that live up to the hype.