Often people think of Dual Axes in Tableau as a way of showing different measures on the same chart, but they can be just as effective, if not more so, in adding an an extra level of information or detail to a chart in a more interesting, or clearer way.
As an example let’s take a look at the Gantt Charts below, they show predicted and actual times for several stages of a project. The second is a lot clearer and enables us to quickly look at the actual vs target/predicted times for each stage.
Similarly dual axes can be used to overlay information such as averages onto charts to overlay on a more detailed, granular level of information – as in the case of the scatter plot below, where the averages per region are shown from the transparent bubbles.
Thirdly we can use dual axes as a way of overlaying information to allow quick interpretation of the information, such as shapes, etc, as in the example below.
Putting together dual axes charts like the above is straightforward, simply pick up the pill you want to duplicate while holding the control key and drag it next to itself on the relevant shelf. Then right click on the second, new pill and select the “Dual axis” option. Straight away you should ensure you also right-click on the new axis and “Syncronize axes”, this ensures everything lines up.
You will notice now, by moving to a dual axes chart, you will have a second “Marks” pane for the second measure, as well as an “All” pane. You can use these panes to change how the two sets of marks are shown, the “All” pane will affect both sets of marks, while the others work independently meaning you can have different types, sizes, colours, etc.
Here’s some top tips for working with dual axes charts like this:
1. Remember to sync Axes – easily the biggest mistake when working with dual axes charts in this way is forgetting to synchronize the axes.
2. Mix up your marks – if things are too similar they will be confusing, ensure you use different marks and also use help text and/or tooltips to give the user guidance in what they are being shown.
3. Don’t add “non-data ink” – whatever you add should be to either give the user new information, or help them interpret the existing data, not just “bling”
4. You may need to reshape your data – e.g. in the first Gantt example we needed to use table calculations to create new dimensions and measures.
5. Use your imagination (and other people’s) – dual axes bring a new dimension you can apply to your visualisations so you can be as as creative as you like, but looking at examples on Viz of the Day and Tableau Public and de-constructing them will help give you new ideas.