The heuristics for grouping commits dosen't play well with any kind of "release" branch or similar branches which follow a master branch with merges from time to time. As it stands, qgit tires to group all these merge commits near the tip of the "release" branch, even if they are years away in the past. This leads to a lot of linking lines running in parallel to the master line over years.
Intended behaviour: there should be a threshhold to stop grouping of commits, e.g. based on an inballance of commits or a time distance
See the example screenshot. Here the "release" branch is the "scons" branch. Note, the tip is in 2011. The grouping combines commits back to 2008, while there are hundreds of interviening commits on master even in 2010