Subplots
When one isn't enough.
When comparing multiple measures it's sometimes helpful to make use of subplots. These create new axes onto which marks can be mapped.
The number of subplots in the visual will change to as necessary accommodate all marks.
Structure
The large-scale structure of a visual is controlled by Subplots, Facets and Secondary Axes

Subplots
In order to change the subplot a given mark is on, click on the ellipsis next to the mark type to bring up the mark controls:

Subplots allow multiple marks to be plotted against a common axis in one direction, with separate scales in the other
Count supports up to 64 (8 x 8) subplots in a visual

Butterfly chart
In the following example of a butterfly chart we use two x-axes to show two different metrics. We use subplots and reverse the scale of one mark to achieve this. These x-axes share the same y-axis.

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