
The assault on the pie chart - sonabinu
http://priceonomics.com/should-you-ever-use-a-pie-chart/
======
scarmig
It seems to me many of these tests are asking pie charts to do something
they're really not meant for. Sure, you can create a synthetic problem and
dataset where you need to order four different categories, all comprising
around quarter of the whole, and there a bar chart would excel.

But that's not actually that common a use case: a lot of the time, I'd only be
interested that they are all roughly a quarter of the whole, and not care
beyond that (providing a ranking of small differences can actually be pretty
misleading, depending on the problem you're trying to answer).

I can just as easily say, I want to know roughly what proportion of the whole
a category comprises. And with widely varying category sizes, the pie chart
excels, while with a bar chart I'd have to sum the bar sizes in my head, and
divide that total by the category in question. That's at least as taxing as
having to differentiate between small differences in pie slice sizes.

The answer isn't "always use pie charts" or "never use pie charts": it's to
use what's best for a particular situation and context.

~~~
stdbrouw
There's no rule that a bar chart cannot mention the proportion of the category
above each bar, and there's also no rule that a bar chart has to be for
absolute and not relative numbers.

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ThomPete
After having worked with a lot of data visualization I came to the conclusion
that the purpose visualizing most data is to show proportion, trend or
significance not details.

I.e. it only has value if it clearly visualizes the point trying to be made.
Otherwise you might as well show it on a list or it might come up as an alert.

The challenge is not to be able to read and understand graphs but to know what
to do with them. This is why most dashboards are purely made to make the
product look sexy not to actually provide any valuable insights.

~~~
mikepmalai
Agreed. Charts are narrative.

They are the visualization equivalent of TLDR. It captures the presenter's
gist of the data but doesn't tell the whole story.

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theophrastus
Here's a (brain) dead simple rule about pie charts: if you're using them to
demonstrate a difference or change, so that you end up showing two, or more,
pies, and if the aggregate amount represented by each pie isn't equal, then
you're up to no good. Even if you try to compensate by making one of the pies
have a larger radius than the other this is bad, as area comparisons will
always be under perceived. If the case distinctions have the same summed
aggregate, and you're just showing how the apportioning of the groups have
changed then you're ok. But then graphically why not just have two bar charts
which are visually more easy to interpret?

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baconner
I build BI products for a living and used to get into a lot of pedantic
arguments about whether a pie chart or worse, the dreaded 3d bar or line plot,
should _ever_ be used. Eventually I had to give it up though as it's not a
productive use of time to try and pry beloved bad visualizations away from
business users, plus I sensed I was starting to become a stephen fewesque
curmudgen. They haven't reasoned their way into liking them so you can't
reason them out of it most of the time.

Instead I'm focusing on building tools that show data in better ways first,
but let you get to other options if you really want to giving viewers you
share views with a way to quickly switch back to something easier to grok.
Even initially graphically purist tools like Tableau have had to go this route
and you can now find nightmarish packed bubble plots and the like in there. In
modern visualization there are so many worse visualizations that have crept in
that arguing about the pie chart seems almost quaint to me at this point. For
example check out this _flying 3d globe with bar chart where the bars point
directly up (at the camera)_ [https://www.microsoft.com/en-us/powerBI/power-
map.aspx](https://www.microsoft.com/en-us/powerBI/power-map.aspx) \- A while
back someone from Microsoft demonstrated this to me and after I gave him a
hard time about it being an improper way plot geographic data he retorted "The
world _is_ round you know!"

edit: formatting

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mungoman2
I have no problem with pie charts. The area chart on the other hand are pure
evil. For example here, how is the percentage of ios over time? Has market
share grown or shrunk from Q3'11 to Q1'14? It's extremly hard to answer this
kind of question except for the lucky ones getting the top or bottom slot.
[http://static1.businessinsider.com/image/537b72f169bedd1e043...](http://static1.businessinsider.com/image/537b72f169bedd1e04384fab-1200-924/smartphoneosmarketshare.png)

~~~
TazeTSchnitzel
The big, big problem with such charts is that the vertical thickness
_perpendicular to the x axis_ of the line is the actual proportion, but your
brain will look at the thickness perpendicular to the gradient.

To put it visually:
[http://i.imgur.com/vZ7uTu1.png](http://i.imgur.com/vZ7uTu1.png)

The pink bars are the percentage market share as your brain reads it. The
green bars are the percentage market share the graph is supposed to show. (The
burning sensation in your eyes is because I made a poor choice of colours.)

If you were to naively look at the thickness of the black area (shown with the
pink bars), it'd look like it was constant. But look at the vertical height
(green bars): whatever this proportion is, the black has started small, grown
massively, then contracted.

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oscilloscope
I've seen pie charts used particularly effectively in two areas of personal
finance:

1) In budgeting apps, to show the distribution of monthly expenses across
categories.

2) In investment tutorials and reports, to show a distribution across asset
classes.

In the case of budgeting, it's true that a pie doesn't capture all variables,
like total expense from month to month. But if you're trying to optimize a
metric, such as savings (which can be included as an expense category to
visualize savings rate), it makes it intuitive to see which budget categories
are throwing you off. Suddenly it's not about reducing expenses, but rather
shifting them between categories of a whole. At 50%, there is a tipping point
where the pie slice becomes concave. If you're trying to push yourself to a
50% savings rate, it's an interesting visual benchmark.

For portfolios, here are a few examples. These are 3d pie charts, perhaps the
most-mocked visualization type of all. But they communicate the information
effectively enough. Repeated pie charts emphasize to the reader the importance
of asset allocation.

[https://www.bogleheads.org/wiki/Lazy_portfolios](https://www.bogleheads.org/wiki/Lazy_portfolios)

What makes pie charts effective is that they are simple and approachable.
There are better graphs for showing a portfolio's size and allocation over
time-- but not for initially communicating the idea of asset allocation and
giving a few examples. Bogleheads is one of the best resources on passive
index investing, and I think it's not an oversight how often pie charts show
up on the wiki.

If anything, I would say visualizing a portfolio as a line chart can be more
deceptive. This is a common technique used to sell mutual funds that have
recently outperformed to naive investors. Line charts can implicitly promise a
future trend which may not materialize.

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simulate
Pie charts aren't perfect but are useful when applied correctly.

Two great things about pie charts:

1\. From the shape you can tell you are looking portions of a whole. For
example, a pie chart of market shares shows all businesses in the category.

2\. You can easily guess the proportion of the segments relative to each
other. For example, in this chart
[http://www.liv.ac.uk/~cll/lskills/WN/pie%20chart1.gif](http://www.liv.ac.uk/~cll/lskills/WN/pie%20chart1.gif)
you can tell that football and rugby make up half the total and cricket,
tennis, athletics, and other make up the other half.

~~~
jameshart
That encourages you to make a spurious connection between football and rugby.
Football and Other also make up a half, but that wasn't what you immediately
spotted.

~~~
hugh4
Well, they're both winter sports while the others (apart from "Other") are
summer sports.

But I take the point -- you only notice that connection because of the way
they happen to be arranged on the graph.

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nemo44x
One warning about pie charts in many visualization applications today is you
are often looking at how the terms in the chart relate to each other. Not over
the set of all terms.

If I want to draw a pie chart of the top-5 terms for some field and the chart
represents just those terms, then the chart represents how they compare
against each other - not necessarily how those top-5 terms compare against the
entire set.

I know, sounds obvious but I've seen data misinterpreted because of this. The
pie chart must represent the entire set and if you're plotting a subset of the
data in the chart and ignoring the rest then your visualization may not say
what you think it says.

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username223
I can only assume this was covered 60 years ago:

    
    
        https://en.wikipedia.org/wiki/How_to_Lie_with_Statistics
    

Misleading charts have long been an important and valuable career skill.

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tcdent
> _In Cleveland’s paradigm, the perceptual task associated with a bar chart is
> judging position on a scale, and the task associated with a pie chart is
> judging angle. They found that the length judgments for the bar chart were
> “1.96 times as accurate as angle judgments” for a pie chart._

In pie charts, I find it much easier to estimate a percentage than estimate
the angle, perhaps just based on value range alone; 100 vs. 360.

I also find estimating percentage to be harder with a bar chart since there is
no reference to the entire pool of data; the encompassing circle in a pie
chart.

~~~
semi-extrinsic
Pie charts are good if you want to say e.g. "Hey! A large parr of our problems
are caused by X!" That's actually what Florence Nightingale used them for when
she popularized them, she showed the British military leaders that "Hey! Most
of our troop deaths are caused by infections in soldiers with minor injuries!
We can fix that!"

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dgudkov
The problem with pie charts is that it's too easy to use them in a wrong way.
I would say the only scenario when a pie chart might be a right form of data
visualization is when you need to display _shares_ of two, maximum three,
components of a total. It means 2-3 data points only. Trying to use pie charts
for anything else will quickly degrade readability. More data points, day-
over-day comparisons, variations with the circle size, grids of pie charts --
all makes things worse.

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codezero
When I give demos of our product, I often show the pie chart and tell people,
half jokingly never to use it, this always produces a good laugh.

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vuldin
Pie charts have merit, especially when used in a dynamic dashboard that
changes based on either selection, filtering, or the passage of time. They may
be over used, but they are not without merit entirely.

~~~
ThomPete
Rarely yes but only if those changes are noticeable which they most often
aren't.

Most dashboards are purely made because they look good. The actual purpose of
a Dashboard is to give a skilled person an overview of the entire system and
see if anything is out of the ordinary and if not whats wrong.

The way most dashboards are used today they are more working like glorified
lists and gives nothing of value to the user.

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pcunite
I love pie charts. Good luck making sense of scatter and bubble charts unless
you're already intimate with the data.

~~~
ghaff
And that's something of the challenge with visualizations. Obviously some
visualizations are just bad. But others are merely complex because they
include a lot of information. This latter sort of visualization won't be
appropriate everywhere--like accompanying a typical newspaper story--but it
might be just the thing to really dive down into data.

A great example of this is actually the Napoleon army graphic that's famous
for being in Tufte's first book. It's a great visualization of what happened
as Napoleon's army went off to Russia and returned. But what it's showing
isn't really immediately obvious (and probably wouldn't be even it if were
redesigned in modern graphical style).

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mdevere
fuck pie charts

