Merge pull request oxyplot#1207 from dpavsrtrl/issue_1206

issue 1206 replace null with item

Author: objorke

Source/OxyPlot/Series/RectangleSeries.cs

@@ -288,7 +288,7 @@ public override TrackerHitResult GetNearestPoint(ScreenPoint point, bool interpo
                             Text = StringHelper.Format(
                             this.ActualCulture,
                             this.TrackerFormatString,
-                            null,
+                            item,
                             this.Title,
                             this.XAxis.Title ?? DefaultXAxisTitle,
                             this.XAxis.GetValue(p.X),