Fix hit distribution graph midpoint rounding not looking great around zero

Not sure this will be accepted and it's likely only ever going to show
in tests, but seems to be a better approach to midpoint rounding for
this case?

osu.Game.Tests/Visual/Ranking/TestSceneHitEventTimingDistributionGraph.cs

@@ -23,6 +23,12 @@ namespace osu.Game.Tests.Visual.Ranking
             createTest(CreateDistributedHitEvents());
         }
 
+        [Test]
+        public void TestManyDistributedEventsOffset()
+        {
+            createTest(CreateDistributedHitEvents(-3.5));
+        }
+
         [Test]
         public void TestAroundCentre()
         {