Value-Suppressing Uncertainty Palettes – UW Interactive Data Lab – Medium

Value-Suppressing Uncertainty Palette

Uncertainty is one of the most neglected aspects of number-based communication and one of the most important concepts in general numeracy. Comprehending uncertainty is hard. Visualizing it is, apparently, even harder.

Last week I read a paper called Value-Suppressing Uncertainty Palettes, by M.Correll, D. Moritz, and J. Heer from the Data visualization and interactive analysis research at the University of Washington. This paper describes an interesting approach to color-encoding uncertainty.

Value-Suppressing Uncertainty Palette

Uncertainty visualization is commonly done by reducing color saturation and opacity.  Cornell et al suggest combining saturation reduction with limiting the number of possible colors in a color palette. Unfortunately, there the authors used Javascript and not python for this paper, which means that in the future, I might try implementing it in python.

Two figures visualizing poll data over the USA map, using different approaches to visualize uncertainty


Visualizing uncertainty is one of the most challenging tasks in data visualization. Uncertain


via Value-Suppressing Uncertainty Palettes – UW Interactive Data Lab – Medium

Investigating Seasonality in a Time Series: A Mystery in Three Parts

Excellent piece (part one of three) about time series analysis by my colleague Carly Stambaugh

Data for Breakfast

Recently, I was asked to determine the extent to which seasonality influenced a particular time series. No problem, right? The statsmodels Python package has a seasonal_decompose function that seemed pretty handy; and there’s always Google! As it turns out, this was a bit trickier than I expected. In this post I’ll share some of the problems I encountered while working on this project and how I solved them.

In attempting to find posts or papers that  addressed quantifying the extent to which the time series was driven by seasonality, every example I came across fell into one of two categories:

  • Here’s a few lines of code that produce a visualization of a time series decomposition.
  • Here’s how you can remove the seasonality component of a time series, thus stabilizing your time series before building a predictive model.

Also, each example started with “Here’s a time series with a seasonal trend.”…

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Evolution of a complex graph. Part 1. What do you want to say?

Screenshot showing two slides. The first one is titled "low within-group variability". The second one is titled "High between-group variability". The graphs in the slides is the same

From time to time, people ask me for help with non-trivial data visualization tasks. A couple of weeks ago, a friend-of-a-friend-of-a-friend showed me a set of graphs with the following note:

Each row is a different use case. Each use case was tested on three separate occasions – columns 1,2,3. We hope to show that the lines in each row behave similarly, but that there are differences between the different rows.

Before looking at the graphs, note the last sentence in the above comment. Knowing what you want to show is an essential and not trivial part of a data visualization task. Specifying what is it precisely that you want to say is the first required task in any communication attempt, technical or not.

For the obvious reasons, I cannot share the original graphs that that person gave me. I managed to re-create the spirit of those graphs using a combination of randomly generated arrays.
The original graph: A 3-by-4 panel of line charts
Notice how the X- and Y- axes are aligned between all the subplots. Such alignment is a smart move that provides a shared scale and allows faster and more natural comparison between the curves. You should always try aligning your axes. If aligning isn’t possible, make sure that it is absolutely, 100%, clear that the scales are different. Slight differences are very confusing.

There are several small things that we can do to improve this graph. First, the identical legends in every subplot are a useless waste of ink and thus, of your viewers’ processing power. Since they are identical, these legends do nothing but distracting the viewer. Moreover, while I understand how a variable name such as event_prob appeared on a graph, showing such names outside technical teams is a bad practice. People who don’t share intimate knowledge with the underlying data will find human-readable labels easier to comprehend, making your message “stickier.”
Let’s improve the signal-to-noise ratio of this plot.
An improved version of the 3-by-4 grid of line charts

According to our task, each row is a different use case. Notice that I accompanied each row with a human-readable label. I didn’t use cryptic code such as group_001, age_0_10 or the such.
Now, let’s go back to the task specification. “We hope to show that the lines in each row behave similarly, but that there are differences between the separate rows.” Remember my advice to always use conclusions as graph titles? Let’s test how such a title will look like

A hypothetical screenshot. The title says: "low intra- & high inter- group variability"

Really? Is there a better way to justify the title? I claim that there is.

Let’s experiment a little bit. What will happen if we will plot all the lines on the same graph? By doing so, we might create a stronger emphasize of the similarities and the differences.

Overlapping lines that show several repetitions in four different groups
Not bad. The separate lines create some excessive noise, and the legend isn’t the best way to label multiple lines, so let’s improve the graph even further.

Curves representing four different data groups. Shaded areas represent inter-group variability

Note that meaningful ticks on the X-axis. The 30, 180, and 365-day marks provide useful anchors.

Now, let us go back to our title. “Low intra- and high inter- group variability” is, in fact, two conclusions. If you have ever read any text about technical presentations, you should remember the “one point per slide” rule. How do we solve this problem? In cases like these, I like to use the same graph in two different slides, one for each conclusion.

Screenshot showing two slides. The first one is titled "low within-group variability". The second one is titled "High between-group variability". The graphs in the slides is the same

During a presentation, I would show this graph with the first conclusion as a title. I would talk about the implications of that conclusion. Next, I will say “wait! There is more”, will promote the slide and start talking about the second conclusion.

To sum up,

First, decide what is it that you want to say. Then ask whether your graph says what you want to say. Next, emphasize what you want to say, and finally, say what you want to say.

To be continued

The case that you see in this post is a relatively easy one because it only compares four groups. What will happen if you will need to compare six, sixteen or sixty groups? I will try answering this question in one of my next posts

C for Conclusion

Karl Popper

From time to time, I give a lecture about most common mistakes in data visualization. In this lection, I say that not adding a graph’s conclusion as a title is an opportunity wasted

Screenshot. Slide deck. The slide says

In one of these lectures, a fresh university graduate commented that in her University, she was told to never write a conclusion in a graph. According to to the logic she was tought, a scientist is only supposed to show the information, and let his or her peer scientists draw the conclusions by themselves. This sounds like a valid demand except that it is, in my non-humble opinion, wrong. To understand why is that, let’s review the arguments in favor of spelling out the conclusions.

The cynical reason

We cannot “unlearn” how to read. If you show a piece of graphic for its aesthetic value, it is perfectly OK not to suggest any conclusions. However, most of the time, you will show a graph to persuade someone, to convince them that you did a good job, that your product is worth investing in, or that your opponent is ruining the world. You hope that your audience will reach the conclusion that you want them to reach, but you are not sure. Spelling out your conclusion ensures that the viewers use it as a starting point. In many cases, they will be too lazy to think of objections and will adopt your point of view. You don’t have to believe me on this one. The Nobel Prize winner Daniel Kahneman wrote a book about this phenomenon.

What if you want to hear genuine criticism? Use the same trick to ask for it. Write an open question instead of the conclusion to ensure everybody wakes up and start thinking critically.

The self-discipline reason

Some people are not comfortable with the cynical way I suggest to exploit the limitations of the human mind. Those people might be right. For them, I have another reason, self-discipline. Coming up with a short, concise and descriptive title requires effort. This effort slows you down and ensures that you start thinking critically and asking questions. “What does this graph really tells?” “Is this the best way to demonstrate this conclusion?” “Is this conclusion relevant to the topic of my talk, is it worth the time?”. These are very important questions that someone has to ask you. Sure, having a professional and devoted reviewer on your prep team is great but unless you are a Fortune-500 CEO, you are preparing your presentations by yourself.

The philosophical reason

You will notice that my two arguments sound like a hack. They do not talk about the “pure science attitude”, and seem to be detached from the theoretical picture of the idealized scientific process. That is why, when that student objected to my suggestion, I admitted defeat. Being a data scientist, I want to feel good about my scientific practice. It took me a while but at some point, I realized that writing a conclusion as the sole title of a graph or a slide is a good scientific practice and not a compromise.

According to the great philosopher Karl Popper, a mandatory characteristic of any scientific theory is that they make claims that future observations might show to be false. Popper claims that without taking a risk of being proved wrong,  a scientist misses the point  [ref]. And what is the best way to make a clear, risky statement, if not spelling it out as a clear, non-ambiguous title of your graph?

Don’t feel bad, your bases are covered

To sum up, whenever you create a graph or a slide, think hard about what conclusion you want your audience to make out of it. Use this conclusion as your title. This will help you check yourself, and will help your fellow scientists assess your theory. And if a purist professor says you shouldn’t write your conclusions, tell him or her that the great Karl Popper thought otherwise.


Meaningless slopes

That fact that you can doesn’t mean that you should! I will say it once again.That fact that you can doesn’t mean that you should! Look at this slopegraph that was featured by “Information is Beautiful”

What does it say? What do the slopes mean? It’s a slopegraph, its slopes should have a meaning. Sure, you can draw a line between one point to another but can you see the problem here? In this nonsense graph, the viewer is invited to look at slopes of lines that connect dollars with years. The proverbial “apples and oranges” are not even close to the nonsense degree of this graph. Not even close.

This page attributes this graph to National Geographic, which makes me even sadder.


In defense of three-dimensional graphs

“There is only one thing worse than a pie chart. It’s a 3-D pie chart”. This is what I used to think for quite a long time. Recently, I have revised my attitude towards pie charts, mainly due to the works of Rober Kosara from Tableau. I am no so convinced that pie charts can be a good visualization choice, I even included a session “Pie charts as an alternative to bar charts” in my recent workshop.

What about three-dimensional graphs? I’m not talking about the situations where the data is intrinsically three-dimensional. Such situations lie within the consensus. I’m talking about adding a third dimension to graphs that can work in two dimensions. Like the example below that is taken from a 2017 post by Deven Wisner.

Screenshot: a 3D pie chart with text "The only good thing about this pie chart is that it's starting to look more like [a] real pie"

Of course, this is not a hypothetical example. We all remember how the late Steve Jobs tried to create a false impression of Apple market share

Steve Jobs during a presentation, in front of a

Having said all that, can you think of a legitimate case where adding the third dimension adds more value than distraction? I worked hard, and I finally found it.


Take a look at the overlapping density plot (a.k.a “joy plot”).

Three joyplot examples

If you think of this, joyplots are nothing more than 3-d line graphs done well. Most of the time, they provide information-rich data overview that also enables digging into fine details. I really like joyplots. I included one in my recent workshop. Many libraries now provide ready-to-use implementations of joyplots. This is a good thing to have. The only reservation that I have about those implementations is the fact that many of them, including my favorite seaborn, add meaningless colors to the curves. But this is a topic for another rant.

Today’s workshop material

Me in front of an audience

Today, I hosted a data visualization workshop, as a part of the workshop day adjacent to the fourth Israeli Data Science Summit. I really enjoyed this workshop, especially the follow-up questions. These questions are  the reason I volunteer talking about data visualization every time I can. It may sound strange, but I learn a lot from the questions people ask me.

If you want to see the code, you may find it on GitHub. The slide deck is available on Slideshare

Me in front of an audience



If you know matplolib and are in Israel on May 27th, I need your help

So, the data visualization workshop is fully booked. The organizers told me to expect 40-50 attendees and I need some assistance. I am looking for a person who will be able to answer technical questions such as “I got a syntax error”, “why can’t I see this graph?”, “my graph has different colors”.

It’s a good opportunity to attend the workshop for free, to learn a lot of useful information, and to meet a lot of smart people.

It’s a win-win situation. Contact me now at