The book on sparklines
Sparklines, a term coined by Edward Tufte, are becoming increasingly popular in Business Intelligence software. Some applications, like Excel (through various add-ins) and QlikView (starting in version 9.0), have the ability to make them, out of the box, while they can be created elsewhere, like Xcelsius, with a bit of creativity.
You've likely seen them before, but do you know when it is appropriate to use them? They're not to be thrown around just because all of the cool data visualization kids are using them.
The background, from Wikipedia:
The term 'Sparkline' was proposed by Edward Tufte for "small, high resolution graphics embedded in a context of words, numbers, images." Tufte describes sparklines as "data-intense, design-simple, word-sized graphics". Whereas the typical chart is designed to show as much data as possible, and is set off from the flow of text, sparklines are intended to be succinct, memorable, and located where they are discussed.
The clearest and most instructive examples, not surprisingly, can be found in one of Tufte's books, Beautiful Evidence.
- Line representing the last n data points
- Data point for most recent reading highlighted in red
- Value of most recent reading in corresponding red type
- Name of metric
- Acceptable/normal range as gray, shaded area
There isn't a single pixel wasted on meaningless or redundant data, embodying Tufte's data-ink ratio. Another way in which he is practicing what he preaches is that all of the data related to each metric is in close proximity, not requiring repeated references to scattered information. Of course, those are Tufte's specs, and different BI companies and the people who have created custom sparkline components may choose to implement them differently.
If you're looking for guidance on the best way to apply them in your applications, I like how Stephen Few succinctly puts it: "Think of them as an enhanced, much more informative substitute for the trend arrows that often appear on dashboards."
For only marginally more space than a trend marker, sparklines provide significantly more information and paint a more complete picture than simple up/down or green/red indicators. The lack of context surrounding trend indicators leaves open the possibility that a positive indicator represents a minuscule uptick at the end of a significant and long-term drop. In other words, when you look at your dashboard for the day and see a green, up arrow for margin %, that means margin % has improved in the most recent period, while it could still be down for the week, month, quarter, or year (Few explains something similar on page 140 of Information Dashboard Design).
While the line obviously represents some period of time, the horizontal, dimensional axis is not labeled. In fact, neither axis is. The reason is that sparklines are meant to show trends and comparisons, not detailed values, like standard line graphs. This helps explain why they are not a substitute for the standard line graph, which can more easily compare multiple dimensions or multiple measures with greater precision.
Finally, see this thread on Edward Tufte's message board for the single longest conversation about sparklines since the dawn of time.