Data (representing the X, Y, and Z coordinates of each point) in this type of graph are represented in 3D space as "deviations" from a specified base-level of the Z-axis.
Deviation plots are similar to space plots. As compared to
space plots , however in deviation plots the "deviations plane" is "invisible" and not marked by the location of the X-Y axes (those axes are always fixed in the standard bottom position). Deviation plots may help explore the nature of 3D data sets by displaying them in the form of deviations from arbitrary (horizontal) levels. Such "cutting" methods can help identify interactive relations between variables.
See also,
Data Rotation (in 3D space) in the
Graphical Techniques chapter.