The historical analysis where a file has changed is not really complete withouth displaying merges. Now if a file is changed is sub-branch and merged back to its pareent branch at some point, this is not visualized in any way. More on this
http://www.plasticscm.net/index.php?/topic/1741-merge-not-displayed-neither-in-file-history-nor-in-file-2d-history/