def error_msg_wx(msg, parent=None):
    """
    Signal an error condition -- in a GUI, popup a error dialog
    """
    dialog =wx.MessageDialog(parent  = parent,
                             message = msg,
                             caption = 'Matplotlib backend_wx error',
                             style=wx.OK | wx.CENTRE)
    dialog.ShowModal()
    dialog.Destroy()
    return None