When radial bars, progress rings and gauges are the right choice, when another chart reads better, and how to draw one that tells the truth. The same rules hold in every chart library. For picking between chart types in general, see Choosing a chart.
What it is for
A radial bar chart bends bars into rings around a center. Each ring is a category, and the angle it sweeps is the value. With one ring it becomes a progress ring: a value against a goal, such as seats used of 25. A gauge reads one value on a partial ring, often against colored bands.
Most of the time a radial chart is a styling choice. It fits a dashboard tile well, but a straight bar shows the same values more accurately. Pick it for the shape only when the reader does not need to compare values closely.
When to use
One value against a goal or a capacity, such as steps toward 10,000 or storage used of the plan.
One reading against a few named ranges, such as a rate that is low, elevated or high.
A compact tile with one to three rings, each labeled with its number.
Use something else when
The reader compares categories or ranks them. Use a horizontal bar chart.
There are more than about five categories. Use a bar chart, which has room for all of them.
The values are parts of one whole. Use a pie, a donut or a stacked bar.
The value changes over time and the trend matters. Use a line chart, or a sparkline next to the number.
There is no goal or range to measure against. Show the number as a statistic.
The progress sits in a list or a table row. Use a progress bar, which fits a line of text.
How it reads
A ring encodes its value as angle. An outer ring is longer than an inner ring for the same angle, so readers see the outer bar as larger even when the values are equal. The eye compares arc lengths, not angles, so concentric rings mislead as soon as the values are close. A single ring or a gauge avoids this, because there is nothing to compare it with.
Do it well
Say what a full ring means. Set the end of the scale to the goal or the capacity, not to the largest value in the data.
Draw a light track for the full range, so the reader sees how much is left.
Write the value as a number, in the center or at the start of each ring. The arc alone is not enough.
Keep a hole in the middle, so the innermost ring does not shrink to a dot.
Sweep less than a full turn when rings are compared, so the largest bar does not close on itself.
In a gauge, use few bands with meaningful limits, and name them.
Pitfalls
Comparing rings by length. The outer ring always looks larger. Label every ring, or use a bar chart.
A value past the end of the scale. The ring wraps around and reads as a small value. Cap it, and say that the goal was passed.
A gauge with no number. The reader estimates an angle. Write the value under the needle.
Bands in red and green only. Readers with color vision deficiency cannot tell them apart. Name the bands, or mark them with ticks.
Many thin rings. The inner ones become too short to read. Keep it to a few rings, or use bars.
Variants
Radial bars: one ring per category, sweeping from a shared start.
Progress ring: one ring and its track, with the value in the center.
Stacked ring: segments placed end to end along one ring, for the parts of a total.
Gauge: a partial ring, often three quarters or half a turn, filled to the value.
Needle gauge: colored bands, ticks and a needle that points at the value.
Compared with similar charts
Against a bar chart: the same values, read more accurately on a straight baseline. Use bars whenever comparison is the job.
Against a progress bar: the same reading, in less space and easier to align in a list. A ring suits a tile with room around it.
Against a pie or donut: a pie splits one whole. Radial bars show separate values that do not add up.
Against a rose chart: a rose grows each category outward from the center, so the value is the radius, not the angle.
Against a statistic: a number alone is clearer when there is no goal to measure it against.