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You are here: Home / Data and maps / Data visualisations / Learn more / Chart dos and don’ts

Chart dos and don’ts

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A list of valuable usability tips and guidelines for improving your charts
Above all else show the data
Edward Tufte 

Do use the full axis and avoid distortion

For bar charts, the numerical axis (often the y axis) must start at zero.  


Do use the full axis for bar charts. Our eyes are very sensitive to the area of bars, and we draw inaccurate conclusions when those bars are truncated.


Wrong

Correct

 

Another bad example shown on BBC UK show “Breakfast”. Did really the men height doubled from 1871 to 1971?


If you need to show data details that are not visible when using the full axis, than the original chart with full axis must be accompanied with a “zoomed in chart”, a so called “panel chart”. See example below



If you have only one category to show, than you can show a portion of the chart by using a line chart in a specific range.

Another suggestion is to “break” the axis, so that part of the axis shows the small values, then another part of the axis shows the large values, with a section of the axis scale removed. Sounds good, but you’ve lost any correlation between the large and small values.

 

More reading http://peltiertech.com/WordPress/broken-y-axis-in-excel-chart/

Making these charts interactive will solve many of the issues stated above. For example the user would be able to mouse over a column and get the exact value, filter out some categories or sort the columns according to their values for easier comparison.


Use consistent intervals on axis (be transparent on data gaps)

Be clear when some data is missing. Explain the reason why is missing. Use the full axis and do not skip values when you have numerical data.

The x-axis in the "wrong example" below has a time-series with inconsistent intervals (missing years 2003 and 2004) giving a distorted view of data over time.

Wrong

Correct

 

Note: Data has not been reported for 2003 and 2004.

 

Remove any visual clutter (increase data-ink ratio, Tufte’s principle)

Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away Antoine de Saint-Exupery
Remove to Improve (the data-ink ratio)

As shown in the example above, it is important to remove any visual clutter like the dark background and the dark grid (non-data-ink) and instead enhance the visibility of the data information part (data-ink), in this case the bars. The grid can be removed or made in a much more subtle style, since it is a supporting tool rather than the data itself.

Read more about Data-Ink ratio and Tufte's principles

Use a clear language and avoid acronyms

Try to use a clear language in your chart title and descriptions. Avoid acronyms like "MS" and use the extended form "Member State" or even better simply "Country". It is ok to use well-known abbreviations like EU or GDP or those your audience understand clearly. 

Tell the "why" and "how"

Most people simply identify what is being measured in the title line or other descriptive information, leaving the reader with no help or clue on how to read or why the chart was made.

Use a descriptive chart title and annotation that not only describe what is being measured rather also why the reader should care and how to read the chart. This will avoid misinterpretation and save time for the chart viewer.
Example


Original title: Cadmium emissions

Improved title with note: Change in cadmium emissions. Note: A reduction of emission is an indication of improved air quality in major European cities.

 

Charts are mostly communication tools. We have already made some reasoning on the "why and how" when we choose the chart type (bar, line, scatter plots etc.). Specific chart types are best at showing specific aspects of the data.

You can skip this rule if you are building a raw "Statistical exploratory charting tool" where user can slice and create any chart they want.

For end-products ready to be consumed by the target audience, you should always explain how to read the chart and the reasoning behind it. Try to be objective and leave out any subjective interpretations.

 

Highlight what’s important, tell one story

Although it is possible to tell hundred stories using a single line chart, it makes a lot of sense to keep the focus on just one story.

Therefore you should highlight just one or two important lines in the chart, but keep the others as context in the background.


Another bad example, with no highlighted story

The above chart remade below in a much better version which highlight the rise and fall of Microsoft. Do you see what has made the difference?



Sort your data for easier comparisons

The bar chart below is a good example, where the chart x-axis is sorted on the y-values not on the alphabetic order of the country names.

It is more important to give emphasis to the data itself and sort the chart by the data attributes, rather than non-data attributes  (for example labels like country names).

It will be otherwise very difficult if not impossible for users to do a proper comparison across the many bars. It is in any case easy with a quick eye-scan to find your own country in the list.

If the chart is interactive, give the user the possibility to change the default sort order and a way to filter out data and compare only a few categories.

The pie chart below (even though pie charts should be avoided) works also better when presented with sorted data values. It starts at 12 o’clock with the largest slice. It is much easier to understand the relations between the parts, what is bigger and what is smaller, even when the values are not readable or the areas are very similar.

Use direct labeling wherever possible, avoiding indirect look-up

If possible label lines individually and avoid legend (Gregor Aisch, Doing the Line Charts Right)

 

Rotate bar chart when category names are too long

Rotate bars if the category names are long (Cole Nussbaumer, my penchant for horizontal bar charts)

 

Do not use legend when you have only one data category

If there is only one value category plotted in your chart, than there is no need to have a legend. The title can already contain all needed information. Otherwise you can label the axis directly.

Chart with a legend that is not needed (before)

chart with no need for legend

The legend display one category only and it is already in the title, no need to add it to the axis either.

Chart after we removed the unnecessary legend information

chart after we removed the unnecessary legend information

Do use proper aspect ratio to minimize dramatic slopes effects

The slope of a line chart should be close to 45 degrees for the best perception. 

Robert Kosara has a great summary of the "banking to 45 degrees" practice first proposed by Bill Cleveland

Here are the examples given by Kosara:

The same data is presented three ways. The slope is a reflection of the scales used on the two axes.

However, in some cases there can be legitimate reasons why not to stick completely to "banking to 45 degrees". For example to analyze the data and reveal certain patterns which would not be visible in the 45 degree slope. See example below.

Two plots of monthly atmospheric carbon dioxide measurements, taken from 1959 to 1990. The first plot, with an aspect ratio of 1.17, reveals an accelerating increase in CO2 levels. The second plot, with an aspect ratio of 7.87, facilitates closer inspection of seasonal fluctuations, revealing a gradual attack followed by a steeper decay. These aspect ratios were automatically determined using multi-scale banking.Source:  Computer Science Division, University of California, Berkeley (http://vis.berkeley.edu/papers/banking/)

Two plots of monthly atmospheric carbon dioxide measurements, taken from 1959 to 1990. The first plot, with an aspect ratio of 1.17, reveals an accelerating increase in CO2 levels. The second plot, with an aspect ratio of 7.87, facilitates closer inspection of seasonal fluctuations, revealing a gradual attack followed by a steeper decay. Source: Computer Science Division, University of California, Berkeley (http://vis.berkeley.edu/papers/banking/)

Do adjust for inflation in long-time series

When using economic values in your charts than you must be carefull about adjusting the value according to inflation.

This is done by using the CPI (consumer price index). A Euro in 2010 just does not have the same spending power as a Euro in 1961. 

Source: http://www.aboutinflation.com/inflation/european-union---inflation

The purchasing power of €100 EUR in year 1961 is equivalent to €1948 EUR in year 2010.

Do ask others for opinions

Have a fresh set of eyes look at what you've done and give you feedback. You may be surprised by what is confusing – or enlightening! – to others.

Don't use 3D or blow apart effects

Studies show that 3D effects reduce comprehension. Blow apart effects likewise make it hard to compare elements and judge areas.

Bad chart examples

Below a very creative 3D-pie chart and very incomprehensible as well.


Below another (in)famous “churtjunk”. Compare the 21,2% with the 19,5% slices in the pie. Which one looks bigger?

(presented by Steve Jobs at Engadget 2008 http://www.engadget.com/2008/01/15/live-from-macworld-2008-steve-jobs-keynote/)

 

Avoid pie charts and donuts

The human mind thinks linearly: we can easily compare lengths/heights of line segments but when it comes to angles and areas most of us can't judge them well. Therefore try to avoid the use of pie charts when comparing a large number of items. Simple pie charts displaying 2-3 categories may work just fine, but when displaying more data it is better to choose another chart type.

It is difficult to compare many slices in a pie chart. Try alternative charts to convey your message, most often bar or column charts will be a much better alternative. 

Pie chart displays United States population by state. This kind of chart does a very bad job in communicating the data. Too many slices. A sorted column/bar chart is a much better alternative for such data.


The pie chart shows United States population by state. However id does a very bad job in communicating the data. There are just too many slices.

A sorted column/bar chart will work much better. The values are much easier to compare in a bar/line chart than in a pie chart. If you need to show parts of a whole (percentages), you can still use a column chart with the y-axis having percentages.

Pie charts do not work well even when displaying a few categories. See the three examples below. In all three examples all slices in the pie charts looks very similar and one cannot precisely distinguish and compare the differences between them. The alternative column charts are superior, clear and display the quantitative data correctly.  

You can't compare angles and slice areas to each other. Human perception and cognition is poor when viewing angles and areas and trying to make a mental comparison. Pie charts overload the working memory, forcing the person to make complicated calculations, and at the same time make a decision based on those comparisons.

 

The donut chart is just another pie chart with a hole punched in the middle. The donut chart is a useless chart made worse. Avoid donut charts for the same reasons.

Further reading:

Avoid stacked charts, difficult for comparing data

Avoid stacked charts since the parts in the stacked charts are difficult to compare with each other.

To solve this issue some chart tools allow the user to filter out interactively the stacked categories and be able to do have a single category displayed.

Same issue applies to stacked areas charts. It is difficult to compare the areas in the different regions when stacked (figure above) and much easier to have them as lines (figure below) and a separate line for total.


Another example on how bad stacked bar charts can be in certain cases



Let’s see how the chart above looks like as a line chart

Now we can clearly see the decline of household category “Married Couples with Children”. Moreover we can more clearly see the trends in the other categories as well.

 

More reading http://junkcharts.typepad.com/junk_charts/2013/05/more-power-brings-more-responsibility.html

Don't confuse correlation with causation

correlation vs causation

Quite often superimposing time series of two different measurements will show a strong correlation. Many things change same way over time. It is an easy mistake to confuse correlation with causation.

For example if you plot two different data series (A and B) on a common time series, you will notice that both follow a similar pattern over time. It is very hard if not impossible to prove that A cause B or viceversa. There are so many third factors that have influence both on A and B that are not plotted on the chart. Many other external factors can be the cause of both A and B changing the same way over time. Only a very large profound statistical-based study on all factors can give some indication of causation, if any exists.

Even correlation can be questionable when seen on a chart. See de-noising data, a method for identify true correlation by removing the time-series data

 

Don’t use maps for everything that has spatial dimension

Even though your data has a geographical dimension, it doesn't automatically mean that it will best be displayed on a map. Choose your chart type wisely.

In fact most data has a geographical dimension if we think about it but it does not always convey new insight when displayed on a map. A very bad map example below, where a huge amount of data is displayed just because it has a location attached to it. However the user does not get any insight from this map. There is no correlation or pattern in this map which we could further investigate.

Bad map example

Good map example

*

History gives us some good examples
The famous map from John Snow 1854, one of the first map data visualization. It is an excellent example of when it is appropriate to use a map for getting insight on data. In this case a strong correlation was found between the cholera outbreaks and the positions of  water pumps. Further investigation confirmed that the water was contaminated and this was the cause of cholera outbreaks concentrated around those areas.

Video - When to *not* use maps

Another bad example of where a map feels “in the way” and making it more difficult to understand the data displayed on it.

source: Nordregio.se

The map above displays where different sectors of High-Tech manufacturing and R&D is located in the Nordic countries together with their sizes (number of employed in sector). The map does pretty well the job of displaying where these jobs are located, but it is useless when used for comparing the different sizes of the circles. Moreover it is implicit that most jobs are located near the large cities like Copenhagen, Malmö, Gothenburg, Oslo, Stockholm and Helsinki. We learn nothing new here.

It would be much more interesting to see the sectors data plotted on a bar chart (see figure below), optionally grouped by country or other regions. It would make it easier to see which region has most jobs in which sector and easier to compare the different sectors sizes with each other, if that is the story we want to tell.

 

 

See also another good example when small multiple chart is the best alternative to a map.

Don't use more than (about) six colors.

Using color categories that are relatively universal makes it easier to see differences between colors.



 

  • Different colors should be used for different categories (e.g., male/female, types of fruit), not different values in a range (e.g., age, temperature).
  • Do not use rainbows for range values
  • If you want color to show a numerical value, use a range that goes from white to a highly saturated color in one of the universal color categories. no rainbows


Example of bad chart, where we use different colors for same measurement

Now redone with a gradient color:


Don’t forget 7%-10% of your male audience (color deficiency)

Remember, 7% to 10% of the male audience have color deficiency issues (color blindness). Therefore make your charts safe against color-blindness.

 

As an example consider the following chart.

Below you have the same chart displayed as a color-blind person would see it. 

Use Vischeck to test your images.

If the chart is readable in black and white than it is even better!

Choose the chart type wisely

Choose the chart type dependently on what you want to show. Get to know the pros and cons of each chart type. 

Onlin wizard helping find the right chart type for the desired communication goal.


Before you start charting, take a step back and ask yourself what are the main questions you want to answer. Choose the right chart type that is best for finding specific patterns and gain possible new insights in your data. Online tools like the Data Visualization Catalogue or a decision diagram [2006, A.Abela] helps you finding the right chart for your data.

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