Fix flaky test PolarPlotTest.testGetLegendItems2

Author: Sujishark

src/main/java/org/jfree/chart/plot/PolarPlot.java

@@ -56,6 +56,7 @@
 import java.io.Serializable;
 import java.util.ArrayList;
 import java.util.HashMap;
+import java.util.LinkedHashMap;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
@@ -258,7 +259,7 @@ public PolarPlot(XYDataset dataset, ValueAxis radiusAxis, PolarItemRenderer rend
         this.axisLocations.put(6, PolarAxisLocation.WEST_ABOVE);
         this.axisLocations.put(7, PolarAxisLocation.SOUTH_LEFT);
 
-        this.renderers = new HashMap<>();
+        this.renderers = new LinkedHashMap<>();
         this.renderers.put(0, renderer);
         if (renderer != null) {
             renderer.setPlot(this);