A pie chart (or a circle chart) is a circular
statistical graphic, which is divided into
slices to illustrate numerical proportion.
In a pie chart, the arc length of each slice
(and consequently its central angle and area),
is proportional to the quantity it represents.
While it is named for its resemblance to a
pie which has been sliced, there are variations
on the way it can be presented.
The earliest known pie chart is generally
credited to William Playfair's Statistical
Breviary of 1801.Pie charts are very widely
used in the business world and the mass media.
However, they have been criticized, and many
experts recommend avoiding them, pointing
out that research has shown it is difficult
to compare different sections of a given pie
chart, or to compare data across different
pie charts.
Pie charts can be replaced in most cases by
other plots such as the bar chart, box plot,
dot plot, etc.
== History ==
The earliest known pie chart is generally
credited to William Playfair's Statistical
Breviary of 1801, in which two such graphs
are used.
Playfair presented an illustration, which
contained a series of pie charts.
One of those charts depicting the proportions
of the Turkish Empire located in Asia, Europe
and Africa before 1789.
This invention was not widely used at first;
The French engineer Charles Joseph Minard
was one of the first to use pie charts in
1858, in particular in maps.
Minard's map, 1858 used pie charts to represent
the cattle sent from all around France for
consumption in Paris (1858).
Playfair thought that pie charts were in need
of a third dimension to add additional information.
It has been said that Florence Nightingale
invented it, though in fact she just popularised
it and she was later assumed to have created
it due to the obscurity of Playfair's creation.
== Variants and similar charts ==
=== 3D pie chart and perspective pie cake
===
A 3d pie chart, or perspective pie chart,
is used to give the chart a 3D look.
Often used for aesthetic reasons, the third
dimension does not improve the reading of
the data; on the contrary, these plots are
difficult to interpret because of the distorted
effect of perspective associated with the
third dimension.
The use of superfluous dimensions not used
to display the data of interest is discouraged
for charts in general, not only for pie charts.
=== Doughnut chart ===
A doughnut chart (also spelled donut) is a
variant of the pie chart, with a blank center
allowing for additional information about
the data as a whole to be included.
Doughnut charts are similar to pie charts
in that their aim is to illustrate proportions.
This type of circular graph can support multiple
statistics at once and it provides a better
data intensity ratio to standard pie charts.
It does not have to contain information in
the center.
=== Exploded pie chart ===
A chart with one or more sectors separated
from the rest of the disk is known as an exploded
pie chart.
This effect is used to either highlight a
sector, or to highlight smaller segments of
the chart with small proportions.
=== Polar area diagram ===
The polar area diagram is similar to a usual
pie chart, except sectors have equal angles
and differ rather in how far each sector extends
from the center of the circle.
The polar area diagram is used to plot cyclic
phenomena (e.g., counts of deaths by month).
For example, if the counts of deaths in each
month for a year are to be plotted then there
will be 12 sectors (one per month) all with
the same angle of 30 degrees each.
The radius of each sector would be proportional
to the square root of the death count for
the month, so the area of a sector represents
the number of deaths in a month.
If the death count in each month is subdivided
by cause of death, it is possible to make
multiple comparisons on one diagram, as is
seen in the polar area diagram famously developed
by Florence Nightingale.
The first known use of polar area diagrams
was by André-Michel Guerry, which he called
courbes circulaires, in an 1829 paper showing
seasonal and daily variation in wind direction
over the year and births and deaths by hour
of the day.
Léon Lalanne later used a polar diagram to
show the frequency of wind directions around
compass points in 1843.
The wind rose is still used by meteorologists.
Nightingale published her rose diagram in
1858.
Although the name "coxcomb" has come to be
associated with this type of diagram, Nightingale
originally used the term to refer to the publication
in which this diagram first appeared—an
attention-getting book of charts and tables—rather
than to this specific type of diagram.
=== Ring chart, sunburst chart, and multilevel
pie chart ===
A ring chart, also known as a sunburst chart
or a multilevel pie chart, is used to visualize
hierarchical data, depicted by concentric
circles.
The circle in the center represents the root
node, with the hierarchy moving outward from
the center.
A segment of the inner circle bears a hierarchical
relationship to those segments of the outer
circle which lie within the angular sweep
of the parent segment.
=== Spie chart ===
A variant of the polar area chart is the spie
chart designed by Dror Feitelson.
This superimposes a normal pie chart with
a modified polar area chart to permit the
comparison of two sets of related data.
The base pie chart represents the first data
set in the usual way, with different slice
sizes.
The second set is represented by the superimposed
polar area chart, using the same angles as
the base, and adjusting the radii to fit the
data.
For example, the base pie chart could show
the distribution of age and gender groups
in a population, and the overlay their representation
among road casualties.
Age and gender groups that are especially
susceptible to being involved in accidents
then stand out as slices that extend beyond
the original pie chart.
=== Square chart / Waffle chart ===
Square charts, also called Waffle Charts,
are a form of pie charts that use squares
instead of circles to represent percentages.
Similar to basic circular pie charts, square
pie charts take each percentage out of a total
100%.They are usually 10x10 grids, where each
cell represents 1%.
Despite the name, circles, pictograms (such
as of people), and other shapes may be used
instead of squares.
The benefit to these is that it is easier
to depict smaller percentages that would be
hard to see on traditional pie charts.
== Example ==
The following example chart is based on preliminary
results of the election for the European Parliament
in 2004.
The table lists the number of seats allocated
to each party group, along with the derived
percentage of the total that they each make
up.
The values in the last column, the derived
central angle of each sector, Is found by
multiplying the percentage by 360°.
*Because of rounding, these totals do not
add up to 100 and 360.
The size of each central angle is proportional
to the size of the corresponding quantity,
here the number of seats.
Since the sum of the central angles has to
be 360°, the central angle for a quantity
that is a fraction Q of the total is 360Q
degrees.
In the example, the central angle for the
largest group (European People's Party (EPP))
is 135.7° because 0.377 times 360, rounded
to one decimal place, equals 135.7.
== Use and effectiveness ==
An obvious flaw exhibited by pie charts is
that they cannot show more than a few values
without separating the visual encoding (the
“slices”) from the data they represent
(typically percentages).
When slices become too small, pie charts have
to rely on colors, textures or arrows so the
reader can understand them.
This makes them unsuitable for use with larger
amounts of data.
Pie charts also take up a larger amount of
space on the page compared to the more flexible
bar charts, which do not need to have separate
legends, and can display other values such
as averages or targets at the same time.Statisticians
generally regard pie charts as a poor method
of displaying information, and they are uncommon
in scientific literature.
One reason is that it is more difficult for
comparisons to be made between the size of
items in a chart when area is used instead
of length and when different items are shown
as different shapes.
Further, in research performed at AT&T Bell
Laboratories, it was shown that comparison
by angle was less accurate than comparison
by length.
This can be illustrated with the adjacent
diagram, showing three pie charts, and, below
each of them, the corresponding bar chart
representing the same data.
Most subjects have difficulty ordering the
slices in the pie chart by size; when the
bar chart is used the comparison is much easier.
Similarly, comparisons between data sets are
easier using the bar chart.
However, if the goal is to compare a given
category (a slice of the pie) with the total
(the whole pie) in a single chart and the
multiple is close to 25 or 50 percent, then
a pie chart can often be more effective than
a bar graph.
The example (left) is of a pie chart with
18 values, having to separate the data from
its representation.
Also several values are represented with the
same color, making interpretation difficult.
The example(right) is of a doughnut shaped
pie chart showing the batting and run records
of Indian Cricket player in test matches of
2019.
Several studies presented at the European
Visualization Conference analyzed the relative
accuracy of several pie chart formats, reaching
the conclusion that pie charts and doughnut
charts produce similar error levels when reading
them, and square pie charts provide the most
accurate reading
