title: wxpython move drawn point with mouse
body: <p>I am using wxpython to do some image processing job now. I want to draw a point with mouse left button down and move the drawn point with mouse move. </p>
  
  <p>The following is my code. It is ok when draw point with left button down, but when I move the mouse the previous drawn points also display which did not meet my expectation. </p>
  
  <p>How to hide the previous point when I move the mouse so that it looks like I am move the points?</p>
  
  <pre><code># -*- coding: utf-8 -*- 
  
  import wx
  
  class MyFrame(wx.Frame):
      isLeftDown = False
  
      def __init__(self, parent):
          wx.Frame.__init__(self, parent, id=wx.ID_ANY, size=wx.Size(500, 500))
          bSizer1 = wx.BoxSizer(wx.VERTICAL)
  
          self.m_panel = wx.Panel(self, wx.ID_ANY, wx.DefaultPosition, 
                                  wx.DefaultSize, wx.TAB_TRAVERSAL)
          bSizer1.Add(self.m_panel, 3, wx.EXPAND | wx.ALL, 5)
  
          bmp = wx.EmptyBitmap(500, 500)
          self.staticBMP = wx.StaticBitmap(self.m_panel, wx.ID_ANY, bmp)
  
          self.SetSizer(bSizer1)
  
          # bind event
          self.staticBMP.Bind(wx.EVT_LEFT_DOWN, self.OnLeftDown)
          self.staticBMP.Bind(wx.EVT_LEFT_UP, self.OnLeftUp)
          self.staticBMP.Bind(wx.EVT_MOTION, self.OnMove)
  
      def OnLeftDown(self, event):
          pos = event.GetPositionTuple()
          dc = wx.ClientDC(self.staticBMP)
          dc.DrawCircle(pos[0], pos[1], 5)
          self.isLeftDown = True
  
      def OnLeftUp(self, event):
          self.isLeftDown = False
  
      def OnMove(self, event):
          if self.isLeftDown:
              pos = event.GetPositionTuple()
              dc = wx.ClientDC(self.staticBMP)
              dc.DrawCircle(pos[0], pos[1], 3)
  
  
  if __name__ == '__main__':
      app = wx.App()
      frame = MyFrame(None)
      frame.Show(True)
      app.MainLoop()
  </code></pre>