Dual axis is a great yet succinct way of showing two sets of measures in contrasting or complementary styles. It is a must have in our viz design arsenal.

I noticed sometimes one may choose to use dual axis while it is not necessary. Here I try to share some thoughts about when to dual axis and when not to.

When not to dual axis

When needing to display two measures in the same chart, intuitively one starts using dual axis. Actually it is possible to display multiple measures using a single axis instead of dual:

Solution 1: Drag measures to one axis
http://onlinehelp.tableau.com/current/pro/online/mac/en-us/tips_multiplemeasures.html

Solution 2: The first solution is quick if you happened to have learnt to do so. For some of us, we may like to do it in logical steps:
- Step 1. Drag the Date dimension to column
- Step 2. Drag Measure Names to filter shelf and in the filter options, pick the measures you want to display
- Step 3. Drag Measure Values to row (you see a single line)
- Step 4. Drag Measure Names to Color shelf (you see one line per measure)



Now it's done. Note that there is a Measure Values shelf appeared after you put it in row. You can drag more measures to the shelf if you want to display more.

It is similar to the method in the article below, and a little different.
http://kb.tableausoftware.com/articles/knowledgebase/dynamicmeasures

When using a single axis, you are supposed to display all measures in a single chart type.

When to dual axis

The most likely usage of dual axis is for the purpose of displaying 2 groups of measures in 2 contrasting or complementary chart types. Thus, it is good to remember

Dual Axis = Dual Chart Types

Below is a chart mixing bar and line comparing sales and running totals (Pareto analysis) based on a single measure. It shows that the top 4 products account for 50% of total sales.



Because each chart type may have subtypes due to different settings, for example wide bars and narrow bars in the same chart, dual axis may be used to mix two subtypes of one same major chart type.

There are 11 major chart types in Tableau:



Caveat for Synchronizing Axis

Sometimes you may not be able to synchronize measures of Integer type. Converting your measures to float can solve the problem. Just use a calculated field:
instead of using [Measure] directly.

Dual axis examples

People are getting creative using dual axis to create great visualizations. Dual axis opens a much uncharted territory in viz design. Here I found a plethora of dual axis designs.

- Bar + Line: it's classic
http://drawingwithnumbers.artisart.org/bars-and-lines/

- Map + Pie
http://kb.tableausoftware.com/articles/knowledgebase/creating-filled-maps-pie-charts

- Bar + Circle: Lollipop chart
http://gravyanecdote.com/andy-cotgreave/lollipop-charts/

- Line + Line: Slope chart
http://gravyanecdote.com/tableau/how-to-make-a-slope-chart-in-tableau/

- Filled Map + Circle
http://www.tableau.com/public/blog/2014/04/dual-axis-maps-2427

- Gantt + Gantt
http://wannabedatarockstar.blogspot.com/2014/05/dual-axis-gantt-chart.html

- Pie + Pie: Donut chart
http://vizwiz.blogspot.com/2014/12/donutcharts.html

- Polygon + Circle
http://blog.databender.net/2014/06/23/points-and-polygons/

- Gantt + Circle: Equiplot
http://healthintelligence.drupalgardens.com/content/equiplot-chart-display-equity-data

- Stacked Bars + Circle: Likert Scales chart
http://www.datarevelations.com/likert-scale-nirvana.html

- (Bar or Line) + Shape: Fun chart
http://www.datarevelations.com/was-stephen-few-right-my-problems-with-a-companys-iron-viz-competition.html

Drop me line if you know a cool dual axis viz. I'd love to know.