Rotating an object on a touch event in kivy

I'm working on making a circle that will spin kind of like a large dial. Currently, I have an arrow at the top to show which direction the dial is facing. I'd like its behavior to be kind of like an old timey rotary phone, such that while your finger/cursor is down you can rotate it, but it'll (slowly) yank back to top after you let go.

Here's what my object looks like:

And here's my code:

#!/usr/bin/kivy
import kivy
kivy.require('1.7.2')
import math

from random import random
from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.gridlayout import GridLayout
from kivy.uix.anchorlayout import AnchorLayout
from kivy.uix.relativelayout import RelativeLayout
from kivy.graphics import Color, Ellipse, Rectangle

class MinimalApp(App):
    title = 'My App'
    def build(self):
        root = RootLayout()
        return(root)

class RootLayout(AnchorLayout):
    pass

class Circley(RelativeLayout):
    angle = 0
    def on_touch_down(self, touch):
        ud = touch.ud
        ud['group'] = g = str(touch.uid)
        return True
    def on_touch_move(self, touch):
        ud = touch.ud
#        print(touch.x, 0)
#        print(self.center)
#        print(0, touch.y)
#        print(touch.x - self.center[0], touch.y - self.center[1])
        y = (touch.y - self.center[1])
        x = (touch.x - self.center[0])
        calc = math.degrees(math.atan2(y,x))
        angle = calc if calc > 0 else (360 + calc)
        print(angle)
    def on_touch_up(self, touch):
        touch.ungrab(self)
        ud = touch.ud
        return True

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

And the kv:

#:kivy 1.7.2
#:import kivy kivy

<RootLayout>:
    anchor_x: 'center'                              # I think this /is/ centered
    anchor_y: 'center' 
    canvas.before:
        Color:
            rgba: 0.4, 0.4, 0.4, 1
        Rectangle:
            pos: self.pos
            size: self.size
    Circley:
        anchor_x: 'center'                          # this is /not/ centered.
        anchor_y: 'center' 
        canvas.before:
            PushMatrix
            Color:
                rgba: 0.94, 0.94, 0.94, 1
            Rotate:
                angle: self.angle
                axis: 0, 0, 1
                origin: self.center
            Ellipse:
                source: 'arrow.png'
                size: min(self.size), min(self.size)
                pos: 0.5*self.size[0] - 0.5*min(self.size), 0.5*self.size[1] - 0.5*min(self.size)
                Label:
                    text: unicode(self.size)    # this is /not/ appearing
                    color: 1,0,0,1
        canvas.after:
            PopMatrix

Parts of that are borrowed from the kivy touchtracer demo, and from this SO question.

You can see I have a calculation that is correctly printing the angle between the origin of the circle and the touch event (not sure how this'll respond to multiple fingers, haven't thought that far through), but not sure how to integrate this into a "spinning" feedback event in the interface.