The Axes settings buttons allow you to :

1. Store a set of axes limits
2. Retrieve a previously stored set of axes limits
3. Toggle the axes shape between square and rectangular
4. Toggle the axes aspect ratio between equal and unequal

As shown above, you can make the axes square and using equal ratios make the grid lines of constant natural frequency circular, as they should be.

Note that the current Axes settings for shape and aspect ratio are shown in red.