plotter: ILinePlot: make _fix_indexes always return a dictionary

Signed-off-by: Michele Di Giorgio <michele.digiorgio@arm.com>
diff --git a/tests/test_plotter.py b/tests/test_plotter.py
index 2ec1fd8..139bea9 100644
--- a/tests/test_plotter.py
+++ b/tests/test_plotter.py
@@ -312,7 +312,7 @@
 
         expected_index = index1 + index2
         expected_index.sort()
-        self.assertEquals(expected_index, merged.index.tolist())
+        self.assertEquals(expected_index, sorted(merged["s1"].keys()))
 
     def test_dygraph_colors(self):
         """Check that to_dygraph_colors() constructs a valid dygraph colors argument"""