When people think of bubble charts, they think of the late great professor Hans Rosling. But what made his most famous data presentations so special? We're going to have a look at the anatomy of a bubble chart by going back to first principles.
The data behind the chart starts life as a simple scatterplot. The most famous example, Rosling uses income on the x-axis and life expectancy on the y-axis. Each dot on the chart represents a country. And we can see that there are some variations between those countries in the top right of the chart and those countries in the bottom left.
But a big part of the story is still hidden. You can see that on the left-hand side of the chart lots of clustering in the lower-income countries. And that's masking important detail. For example, the difference between $500 at lower-income levels is much more important than the difference of $500 in higher-income levels.
So what can we do to fix this? Rosling cleverly transformed the x-axis scale from this to this, a log-scale chart. And by doing so, we can now see the variations in lower income countries that were previously hidden. And in fact, we can see variations between the countries at all income levels.
The next stage in the chart's development is to focus on the dots themselves. Each dot represents a country but the population of each country is very different. So Rosling transforms the size of these dots to represent that population level.
Suddenly, we can not only see the population of each country but also the distribution of income and life expectancy across the globe. Being able to see the size of country populations is useful but a clever twist of Rosling's is to introduce colour to show us where those countries are without putting them on a map.
Now we can see that, for example, in sub-Saharan Africa, people are generally poorer and lead shorter lives than in other regions, for example, America.
The final stroke of genius in Rosling's charts was his recognition of how countries have changed over time. It's such an important part of the story and he introduces that onto the chart by using animation. And now we can see some extraordinary stories. For example, China powering away as it overtakes other countries-- its citizens leading longer lives and experiencing much higher levels of income.
Contrast that with the Democratic Republic of Congo, which goes on a very different trajectory. The animation also shows something the Rosling was very passionate about-- that the world is getting better. And we can see on the chart that the bubbles rise upwards and to the right, which shows us the people in the world are generally getting wealthier and living longer.