One very important feature in Tableau is the ability to use a dual axis. Scatterplots use two axes, but they are X and Y. You also observed in the stacked bar example that placing multiple continuous (green) fields next to each other on Rows or Columns results in multiple side-by-side axes. Dual axis, on the other hand, means that a view is using two axes that are opposite each other with a common pane.

Here is a sample view using a dual axis for Sales and Profit:

There are several key features of the view, which are as follows:

• The Sales and Profit fields on Rows indicate that they have a dual axis by sharing a flattened side.
• The axes defined by Sales and Profit are on opposing sides of the view. Also, note that they are not synchronized, which, in many cases, can give a distorted view of the data. It would be great if profit was that close to total sales! But it's not. To synchronize the axes, right-click the right axis and select Synchronize Axis. If that option is grayed out, it is likely that one of the values is a whole number type and the other is a decimal type. You can change the data type of one of the fields by right-clicking it in the data pane and selecting Change Data Type | Number (Whole) or Number (Decimal).
• The Marks card is now an accordion-like control with an All section and a section for Sales and Profit. You can use this to customize marks for all measures or specifically customize marks for either Sales or Profit.

To create a dual axis, drag and drop two continuous (green) fields next to each other on Rows or Columns, then use the drop-down menu on the second, and select Dual Axis. Alternatively, you can drop the second field onto the canvas, opposite the existing axis.

A combination chart extends the use of dual axes to overlay different mark types. This is possible because the Marks card will give options for editing all marks or customizing marks for each individual axis.

As an example of a combination dual axis chart, consider the following visualization:

This chart uses a combination of bars and lines to show the total number of patient visits over time (using bars) and the breakdown of patient visits by hospital branch over time (using lines). This kind of visualization can be quite effective at giving additional context to detail.

There are several things to note about this view:

• The field on the Color shelf is listed as Multiple Fields and is gray on the Marks card. This indicates that different fields have been used for Color for each axis on Marks.
• The view demonstrates the ability to mix levels of detail in the same view. The bars are drawn at the highest level (patient visits for each month), while the lines have been drawn at a lower level (patient visits for each branch for each month).

• The view demonstrates the ability to use the same field (Patient Visits, in this case) multiple times on the same shelf (Rows, in this case).
• The second axis (the Patient Visits field on the right) has the header hidden to remove redundancy from the view. You can do this by unchecking Show Header from the drop-down menu on the field in the view or right-clicking the axis or header you wish to hide.

Dual axis and combination charts open up a wide range of possibilities for mixing mark types and levels of detail, and are very useful for generating unique insights. We'll see a few more examples of these throughout the rest of this book, but definitely experiment with this feature and let your imagination run wild with all that can be done.