display MatplotlibViewer with retina as appropriate

Addresses usnistgov#584

fipy/viewers/matplotlibViewer/matplotlibViewer.py

@@ -58,6 +58,30 @@ def _isnotebook():
     except (ImportError, NameError):
         return False      # Probably standard Python interpreter
 
+def _isretina():
+    """return True if jupyter notebook configuration
+    InlineBackend.figure_formats contains 'retina' or
+    InlineBackend.figure_format is set to 'retina'
+    """
+    isretina = False
+
+    try:
+        import IPython
+
+        cfg = IPython.get_ipython().config
+
+        if "InlineBackend" in cfg:
+            inbk = cfg["InlineBackend"]
+            if "figure_formats" in inbk:
+                if "retina" in inbk["figure_formats"]:
+                    isretina = True
+            if not isretina and ("figure_format" in inbk):
+                isretina = inbk["figure_format"] == "retina"
+    except ImportError:
+        pass
+
+    return isretina
+
 class AbstractMatplotlibViewer(AbstractViewer):
     """
     .. attention:: This class is abstract. Always create one of its subclasses.
@@ -201,10 +225,14 @@ def _repr_png_(self):
         
         Invoke with `display(myViewer)`
         """
-        from IPython.core.pylabtools import print_figure
-
-        self.plot()
-        return print_figure(self.axes.get_figure(), "png")
+        from IPython.core.pylabtools import print_figure, retina_figure
+
+        fig = self.axes.get_figure()
+
+        if _isretina():
+            return retina_figure(fig)
+        else:
+            return print_figure(fig, "png")
 
 class _ColorBar(object):
     def __init__(self, viewer, vmin=-1, vmax=1, orientation="vertical"):