work around issue #278 (broken qt5agg on windows)

Author: stevengj

src/PyPlot.jl

@@ -121,8 +121,13 @@ function find_backend(matplotlib::PyObject)
     qt2gui = Dict("pyqt5"=>:qt_pyqt5, "pyqt4"=>:qt_pyqt4, "pyside"=>:qt_pyside)
 
     rcParams = PyDict(matplotlib["rcParams"])
-    default = lowercase(get(ENV, "MPLBACKEND",
-                            get(rcParams, "backend", "none")))
+    default = lowercase(get(ENV, "MPLBACKEND", "none"))
+    if default == "none"
+        default = lowercase(get(rcParams, "backend", "none"))
+        if is_windows() && default == "qt5agg"
+            default = "qt4agg" # workaround issue #278
+        end
+    end
     if haskey(matplotlib2gui,default)
         defaultgui = matplotlib2gui[default]