Kivy: centered grid

centergrid.py

import kivy
kivy.require('1.8.1')

from kivy.app import App
from kivy.lang import Builder

root = Builder.load_string('''
BoxLayout:
	orientation: 'vertical'
	Label:
		size_hint_y: None
		height: dp(50)
		text: 'Label'
	
	AnchorLayout:
		id: anchor_layout
	
	Button:
		size_hint_y: None
		height: dp(50)
		text: 'Button'
		on_press: app.add_grid()
''')

class TestApp(App):
	def build(self):
		return root

	def add_grid(self):
		grid = GridLayout()
		# ... set up grid layout here
		root.ids.anchor_layout.add_widget(grid)

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