What Does a Bubble Map Show?
A bubble map is a type of geographic chart that represents data values with bubbles. Its a combination of both a bubble chart and a map visuals. Instead of plotting the values on and X, and Y Axis the bubbles are plotted over a geographic area that is represented by the data.
A bubble map shows geographic trends with a key measure. We use the bubble sizes to spot trends. Meaning larger and smaller bubbles will give us a feel of areas that stand out.
For more charting tips check out my posts on Choropleth Maps, Pie Chart Alternative , Radial Charts.
A simple example…
The example below, shows the unemployment rate for each state. Small bubbles
(low unemployment) like Utah, and Idaho, as well as large bubbles (High unemployment) like California, and Nevada stand out fairly quickly.
The level of detail depends on the data available. Bubble maps can show the longitude and latitude, and can also show higher geographic regions. For a rule of thumb, the lower level charts are more powerful.
It also depends on which level of aggregation makes sense, and is most accurate. Since it is a map chart, more populated areas influence higher level aggregation.
Variations of Bubble Maps
Lets considers the chart above the original starting point. We can show the bubbles in different ways so we have a few different variation of bubble maps. In the chart above, the unemployment rate is shown by changing the size of the bubble. Every single bubble is a different size growing with larger percentages.
Bubble Bucket Ranges Instead of Each Bubble Being its Own Size
Instead of having the bubbles range in individual sizes across all the values, we can have the values bucketed in ranges. This will help alleviate the risk of outliers. Because the size is determined by the largest and smallest value an outlier will effect the size of all bubbles.
To avoid this, we use ranges to capture values that are significant to the analysis. Then, a legend identifies the bubble size.
On the other hand, if you want to be able to identify the extreme values that maybe harder to do in this versions of the Bubble Map.
Heat Map Combined with the Bubble Map
This variation is pretty interesting because it uses two visual indicators on the same metrics. Its a combination of a choropleth map, and bubble map.
Similar to the first map. The values drive the bubble size. Additionally, the coloring of the bubbles also relates to the values.
This version is helpful because size isn’t always the easiest to interpret. Therefore adding a color component to a bubble chart will strengthen visual trends.
The downside here is that we cant really see the smaller values because of how faint the color is. There are workaround to mitigate this issue. For example, the color palette for the heat map can start with a darker color.
Combining a Heatmap with Bubble Ranges
This is another version of the heatmap bubble chart while using the bucketed ranges. For this heatmap the lowest values are visible because of the color selections. Its a light blue instead of a white color above. The high and mid range values are also really easy to see. This would be one of my preferred bubble map options.
Common Bubble Map Issues
Bubble maps are not perfect. They do have a flaws to consider. Its important to consider the downsides before making a decision to use the chart.
Bubbles Overlapping Areas
One of the larger issues with bubble maps are the sizing of the bubble. When the value makes the bubble too large it overlaps multiple geographies. This makes it difficult to determine what are the bubble belongs to.
Using the state map as an example, states that are closer together see this issue more actively. We will focus on the North East US for this example.
Zooming in on the chart above, there are bubble that are difficult to interpret. Some of the bubbles touch three different states. Yes, the software will show data tips to distinguish the states, but good visual should be able to stand alone without the support of software.
Adjustments could be made to bubble sizes while working in the chart.
Bubbles Cover the Map and Useful Border, and Labels
Ill keep the using the example above. The size and color of the bubbles covers most of the state information. A reader who isnt as knowledgeable of the area wont know how to read the chart.
Where can I make a Bubble Map?
bubble charts are quite complex to make in excel. Its more a work around from available features. The choropleth map is really easy to create in Excel though.
Tableau, and PowerBI have pretty easy functionality to create bubble maps.