I am a little confused by what you mean by focusing on "horizontal or vertical segments" - those point to areas where precision is either affected greatly (vertical) or not affected (horizontal) by changes in recall, which would indicate something about the underlying ratio of true vs. false results? I.E. a sharp change means that the change in recall introduced "wrong results" (false positives), while a flat change means the increased recall introduced "true results" (true positives)?