ไม่สามารถวาดใน Clock Callback ใน Kivy

import kivy
kivy.require('1.0.6')

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.graphics import Color, Ellipse


class MyPaintWidget(Widget):
    def on_touch_down(self, touch):
        with self.canvas:
            Color(1, 1, 0)
            d = 30.
            Ellipse(pos=(touch.x - d/2, touch.y - d/2), size=(d, d))


class MyPaintApp(App):
    def build(self):
        return MyPaintWidget()


if __name__ == '__main__':
    MyPaintApp().run()
import kivy
kivy.require('1.0.6')

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.graphics import Color, Ellipse
from kivy.clock import Clock


class MyPaintWidget(Widget):
    def update(self, t):
        with self.canvas:
            Color(1, 1, 0)
            d = 30.
            Ellipse(pos=(200 - d/2, 200 - d/2), size=(d, d))


class MyPaintApp(App):
    def build(self):
        m = MyPaintWidget()
        Clock.schedule_interval(m.update, 1.)
        return m


if __name__ == '__main__':
    MyPaintApp().run()