Bump chart is a two-dimensional ranking visualization tool. It allows us to visualize data rank by one dimension which changes along the other dimension. One example is the soccer league ranking by team along weekly schedule. Here is a video that teaches us how to create a bump chart:
http://www.tableausoftware.com/learn/tutorials/on-demand/bump-charts-0
The Limitations of Bump Chart
Bump chart has its own limitations: When there are a few teams, it is easy to see and track the rank changes. However, when there are a lot of teams, the chart tends to look like spaghetti, or a piece of modern art. Sometimes I would like to view only one spaghetto, that is, one team's ranking along the weeks. This blog will provide a solution for doing just that. Hopefully, the technique presented here will apply to using bump charts in cases of ranking large dimensions.
The following is a bump chart that ranks 51 states (including DC) per each major cause of death. There are 24 major causes of death in total being considered. It seems too much for bump chart to handle.
We can somehow use the color legend to highlight each state's ranking. But the long list of states can't be minimized as a drop down menu and look quite cumbersome. So it is not ideal to use the color legend as a filter.
Pre-Filter vs Post-Filter (the jedi filter)
Filtering a bump chart is not so straightforward. If we use "State" as a quick filter, the entire ranking will be affected because the data set is filtered before ranking. The "State" filter is a pre-filter acting on the data source. Actually, what we need is post-filtering the ranking result. This can be accomplished by creating a magical Jedi filter:
Lookup(min(State),0)
This is a calculated field created from the dimension "State". Using the jedi filter, we are able to select to view or highlight just a few states of interest. We name it "State Filter".
In most cases, we can directly drag the jedi filter to the filter shelf. It can be used as a single-value or a multi-value quick filter. The viz is as follows where we can select to view one or a few of states:
In our case, we will use an alternative filter by parameter, because we want to exclude a blank option due to the fact that some of the causes are major only in a few states.
Thus, we need to create a parameter "Choose a state" that acts as a drop down menu:
And a boolean filter "State filter calc" matches the parameter with "State Filter".
And we will get the following viz that allows us to view one state at a time:
Using circles in Bump Chart
Lines or spaghetti are most used in bump chart. They can be cumbersome sometimes. In the US death cause project, I found that circles or big dots can be a cleaner alternative mark to lines. See the resulting viz as follows:
Hopefully, the technique presented here will expand the usage of bump chart in circumstances where there are a lot of data to be ranked. We can do it by filtering those spaghetti.
(Refresh the page if you want to view the gif image multiple times. Or go to Tableau Public and click the button at the top-right corner.)
The key idea is to float a semi transparent worksheet on top of the dashboard, where a help text box is strategically placed on top of each chart. This way, we can explain how to view each chart and what data points are important, etc. This worksheet is collapsible by a show/hide button.
Below I would like to show how this worksheet can be constructed.
1. Sheet with a single data mark.
- Double click the empty space in Marks panel and add two single quotes. Make the null pill a text label. This creates a single null mark.
- Set the view as "Entire View"
2. Create an show/hide button
- Go to the target dashboard
- Drag a floating vertical container to the dashboard, making it cover all the area of interest.
- Drag the Single Null Mark sheet and drop it into the above container. Hide the sheet title.
- Create an open/close button for the container and place the button at the top-right corner.
3. Add annotations
- Format the sheet background opacity as 70% in the layout manager
- Select area annotations and place them anywhere of interest.
- Write help text and format it to highlight important messages.
- The text can serve as functional guide and/or insight guide.
Here is an example. Feel free to download the workbook and explore. Click the "i" button at the top-right corner to view the in-place help.
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