Add a warning message if pie chart has more than one data set

ChartsOrg/Charts#3286

Author: danielgindi

MPChartLib/src/main/java/com/github/mikephil/charting/data/PieData.java

@@ -1,6 +1,8 @@
 
 package com.github.mikephil.charting.data;
 
+import android.util.Log;
+
 import com.github.mikephil.charting.highlight.Highlight;
 import com.github.mikephil.charting.interfaces.datasets.IPieDataSet;
 
@@ -46,6 +48,18 @@ public IPieDataSet getDataSet() {
         return mDataSets.get(0);
     }
 
+    @Override
+    public List<IPieDataSet> getDataSets() {
+        List<IPieDataSet> dataSets = super.getDataSets();
+
+        if (dataSets.size() < 1) {
+            Log.e("MPAndroidChart",
+                    "Found multiple data sets while pie chart only allows one");
+        }
+        
+        return dataSets;
+    }
+
     /**
      * The PieData object can only have one DataSet. Use getDataSet() method instead.
      *