Pie chart
From Free net encyclopedia
A pie chart is a circular chart divided into segments, illustrating relative magnitudes or frequencies. In a pie chart, the arc length of each segment (and consequently its central angle and area), is proportional to the quantity it represents. Together, the segments create a full disk. A chart with one or more segments separated from the rest of the disk is called an exploded pie chart.
The earliest known pie chart is generally credited to William Playfair's Statistical Breviary of 1801.
Polar area diagram
Florence Nightingale is credited with developing an early form of the pie chart which she called the "coxcomb" which she first published in 1858. This form of pie chart is now known as the polar area diagram, or occasionally the Nightingale rose diagram. The polar area diagram is similar to a usual pie chart, except that the segments are each of an equal angle and differ rather in how far each segments extends from the centre of the circle. It has been suggested that most of Nightingale's early reputation was built on her ability to give clear and concise presentations of data.
Example
The following example chart is based on the results of the election for the European Parliament in 2004. The following table lists the number of seats allocated to each party group, along with the percentage of the total that they each make up. The values in the last column, the central angle of each segment, is found by multiplying the percentage by 360°.
| Group | Seats | Percent (%) | Central angle (°) |
|---|---|---|---|
| EUL | 39 | 5.3 | 19.2 |
| PES | 200 | 27.3 | 98.4 |
| EFA | 42 | 5.7 | 20.7 |
| EDD | 15 | 2.0 | 7.4 |
| ELDR | 67 | 9.2 | 33.0 |
| EPP | 276 | 37.7 | 135.7 |
| UEN | 27 | 3.7 | 13.3 |
| Other | 66 | 9.0 | 32.5 |
Two (or more) pie charts comparing similar data can be created by ensuring that the are drawn to the same scale, using the following formula:
- (radius of b)² / number of data in b = (radius of a)² / number of data in a