code: def on_mouse_move(self, event):
      x, y = event.pos
      h = self.size[1]
      self.data["a_position"][self.index] = x, h - y
      self.data["a_size"][self.index] = 5
      self.data["a_fg_color"][self.index] = 0, 0, 0, 1
      self.index = (self.index + 1) % 500
  
file_path: vispy/examples/demo/gloo/rain.py
imports: import numpy
  from vispy import gloo, app
  from vispy.gloo import Program, VertexBuffer
  from vispy.util.transforms import ortho
label_desc: Use image node_id 6 for calling the Canvas obj's underlying member method code with example usage: obj.on_mouse_move(event) without return types