You open this view by clicking the "ab diagram" tab. In principle, this view is equivalent to the "ab diagram" in "Compare" (see 'ab diagram' View).
On the ab plane, the print process in the L*a*b* color space is viewed from above, i.e. from the direction of white, and the lightness axis L* is perpendicular to the red-green a* axis and the yellow-blue b* axis. The closer a color is to the edge of the diagram, the greater its chroma. The angle indicates the hue. Every color mixture can be defined uniquely and displayed in the L*a*b* color space.
In the ab diagram, lines join up the locations of primary and secondary colors and give you an idea of the size of the color gamut.
The "ab diagram" view is available only if "Show ab diagram in measured data evaluation" is enabled in "Preferences > General" (see 'Preferences' Dialog).