Don't try to re-setup any GUI event loop

Author: tkf

src/init.jl

@@ -48,6 +48,13 @@ end
 
 # return (backend,gui) tuple
 function find_backend(matplotlib::PyObject)
+    if haskey(matplotlib, :pyplot)
+        # If matplotlib.pyplot exists, it means that matplotlib
+        # backend is configured.  It may mean that PyPlot.jl is called
+        # via PyJulia.  Don't setup any GUI event loop in this case.
+        return (matplotlib[:get_backend](), :none)
+    end
+
     gui2matplotlib = Dict(:wx=>"WXAgg",:gtk=>"GTKAgg",:gtk3=>"GTK3Agg",
                           :qt_pyqt4=>"Qt4Agg", :qt_pyqt5=>"Qt5Agg",
                           :qt_pyside=>"Qt4Agg", :qt4=>"Qt4Agg",