When a polar chart is 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 polar chart places data on a circle: the angle is a direction or a position in a cycle, and the distance from the center is the value.
A rose chart, or polar area chart, gives each category an equal wedge and grows it outward to its value: orders by weekday, wind by direction.
A polar line follows a value around a cycle, such as the daily high temperature over a year.
A polar scatter plots points by direction and size, such as wind readings by heading and speed.
When to use
The angle is circular in itself: a compass heading, an hour of the day, a day of the year.
The end of the cycle meets the start, and the reader should see the seam close, such as December next to January.
The reader looks for a shape or a direction: which way most readings point, which season peaks.
Use something else when
The categories have no circular order, such as products or regions. Use a bar chart.
The reader compares values closely. Use a bar chart or a line chart, where values share a straight baseline.
The data runs over time without repeating. Use a line chart.
You compare several years of the same cycle. Use a line chart with one line per year, on a shared month axis.
The parts add up to a whole. Use a pie or a stacked bar.
You compare several measures for one item. Use a radar chart.
How it reads
In a rose chart the value sets the radius of a wedge, but the reader sees its area. Area grows with the square of the radius, so a wedge twice as long looks four times as large. Large values look larger than they are, and small ones nearly vanish.
In a polar line or scatter, the center is not always zero. A temperature scale may start at -10, so distance from the center is a position on the scale, not an amount.
Do it well
In a rose, make area match the value: map the value through a square root scale, or tell the reader to compare lengths and label each wedge.
Start at 12 o'clock and go clockwise, like a compass or a clock. Label the main positions: north, east, south, west, or the months.
Keep categories in their circular order. Sorting the wedges of a rose by value breaks the cycle.
Draw a few light rings with their values, so the reader can read a distance without hovering.
Start the radius at zero for amounts. Start it elsewhere only for measures with no true zero, such as temperature, and label the rings.
Pitfalls
Reading a rose by area. The largest wedge dominates far more than its value. Use a square root scale, or a bar chart.
An inner hole in a rose. Lengths still count from the hole, but areas no longer match the values at all.
A circle for data that does not repeat. The last point lands next to the first and suggests a link that is not there.
Rings without values. The reader cannot tell how far out a point is.
Several polar lines on one chart. They cross near the center, where everything is cramped. Use small multiples or a line chart.
Variants
Rose, or polar area: one wedge per category, its length set by the value.
Stacked rose: wedges split into series, such as wind speed bands per direction.
Polar line: one value followed around a full cycle.
Polar scatter: points placed by direction and magnitude.
Radial bars: bars that sweep around the circle instead of growing out of it.
Compared with similar charts
Against a bar chart: the same values on a straight baseline, read more accurately. A rose only adds the circular order.
Against a line chart: a line chart shows trends and compares cycles better. A polar line shows the seam where a cycle closes.
Against a pie: a pie splits one whole by angle. A rose keeps the angles equal and puts the value in the radius.
Against a radar chart: a radar puts a different measure on each spoke. A polar chart puts one measure around a cycle.