1. A method comprising: accessing reference data describing a diagnostic target for evaluating print system performance, wherein the diagnostic target comprises a plurality of color patches; wherein the reference data specifies expected color values for each of the plurality of color patches in the diagnostic target; accessing measured color values for each of the plurality of color patches in a printed copy of the diagnostic target, the printed copy having been printed by a particular print system; determining a set of difference values by performing a comparison between a measured color value of each patch in the plurality of color patches and the expected color value of the patch; identifying a group of two or more patches in the plurality of patches based on a comparison of the difference values to normal difference values for at least an area of the diagnostic target; wherein the group of two or more patches form a spatial cluster, spatial pattern, or directional trend; wherein the method is performed by one or more computing devices.