Chart Makeover: Line to Lollipop

By Lindsay Lamb

A few weeks ago, we talked about one of the big controversies in data visualization – the pie chart. When used properly, pie charts can be a great way to display certain types of information. Sometimes it’s hard to know what those rules are, and which graph should be used where.

Are there guidelines on the best ways to present data? The answer is yes –  there are many great tools, like Stephanie Evergreen’s chart chooser, that we often use to help me make these kinds of decisions.

One rule of thumb we use in our data visualizations is to only use line graphs when displaying time-related data.

Here’s an example based on confirmed Covid-19 cases (as of 4/1/20):

Confirmed cases for selected countries

Horizontal axis shows the number of days since the case count exceeded 500 in each country

These data are perfect for a line graph. Readers naturally start reading the graph from left to right; without reading anything else we can quickly understand:

• the number of cases are increasing over time.
• some lines have a steeper slope compared to others,
• some countries have had confirmed cases of Covid-19 for a longer period of time than other countries.

We can visualize in our mind projections of each line (unfortunately for us here in the United States, that line seems to be trending straight upward, but who knows! Maybe you are reading this blog in the future and are saying, “What are they so worried about?!? We found a cure, and everyone was fine!” Let’s hope so).

What would the data look like if it were presented differently? Let’s look at the same data in column chart format (updated 4/1/20).

This graph just doesn’t tell the same story. We don’t get the same sense of time, or quite frankly, urgency. We cannot see how some countries flattened the curve compared to other countries. We cannot see where the U.S. is in terms of our trajectory compared to China and Italy. Sure, this graph is informative, and we do see that South Korea has far fewer cases over time, compared to other countries, but the power in that story is gone. Having the lines, slopes, and trajectories for each country in one graph together tells a much more compelling story. This is exactly why line graphs, or time series graphs, are so powerful. The time series graph created by Johns Hopkins CSSE contains a lot of valuable information all in one place (if you like data, I highly recommend taking a look at their dashboard). You can look at the graph and understand the story at first glance, without having to do much reading.

Now let’s walk through an example from a report we were working on with one of our clients. In this example, a line graph did not make sense with the data we had.

When I looked at this figure, as with the Johns Hopkins figure above, I started reading the data from left to right, and assumed the data progressed over time. Instead, the data were based on categories of information (e.g., scores on different components of the ACT) and displayed scores for treatment and control group students in each category. It was difficult to compare the two groups within each category. I was spending too much time thinking about the data within the figure, and if I was thinking too much, our clients certainly would be as well.

I talked to Andrea about the graph (I’m calling her out – it was her graph, originally) – and she said that she had considered a column chart, but it just wasn’t having the same impact as a line chart.

We needed a different visualization.

I reworked the data using one of my favorite graphs I learned from Stephanie Evergreen – a lollipop chart!

Now, the reader can look at the data and quickly compare the two groups (i.e., treatment and control) within each category. It is easy to see that the treatment students outperformed matched control group peers in each subject.

Want to know how to make a lollipop chart? Check out Stephanie Evergreen’s website where she walks you through an example. This is the type of skill we include in our telling stories with data training we are excited to offer. This training is available online so if you are looking to hone your skills while quarantined at home, let us know!