Matplotlib slider widgets (user-defined script-generated) aren't triggering recalculation and plot update

Here is a highly abstracted main program and module. Matplotlib slider widgets should cause the Data instances to recalculate and the plot should then update.

Each time a slider is updated, it should pass its new value to the appropriate method defined during the slider's instantiation. For example, moving the first slider should send it's value to d1.set_a() which triggers a recalculation of that data, and should then trigger P.offsets() (see MODULE) to update the plot.

Question: How can I get these user-defined, script-generated Sliders to trigger the data objects and the plot to update? Do the Slider Widgets instances offer more convenient methods than the way I'm doing it here?

MAIN Program:

import numpy as np
from MODULE import Data, Plot

x0 = np.linspace(0, 10., 11)
y1, y2 = [0.5 * (1.0 + f(x0)) for f in (np.cos, np.sin)]

d1, d2 = Data('hey', x0, y1), Data('wow', x0, y2) # data generating objects

p = Plot('hey')    # plot object

p.add_slider(d1, 'set_a', d1.a, (0.2, 1.0))
p.add_slider(d1, 'set_p', d1.p, (0.5, 2.0))
p.add_slider(d2, 'set_a', d2.a, (0.2, 1.0))
p.add_slider(d2, 'set_p', d2.p, (0.5, 2.0))

p.plotme((d1, d2))

Current result, sliders move but don't trigger recalculation/replotting:

MODULE:

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.widgets import Slider

class Plot(object):
    def __init__(self, name):
        self.name = name
        self.sliders = []
        self.axcolor = 'lightgoldenrodyellow'
        self.fig = plt.figure()

    def sliderfunc(self, value):
        print('sliderfunc')       # currently not getting here
        for (obj, P) in self.OPs:
            P.set_offsets(obj.xy.T)
        self.fig.canvas.draw_idle()

    def add_slider(self, obj, method, nominal, limits):
        ybot = 0.03 * (len(self.sliders) + 1)
        name = obj.name + '.' + method 
        ax_slider   = plt.axes([0.25, ybot, 0.50, 0.02], facecolor=self.axcolor)
        slider      = Slider(ax_slider, name, limits[0], limits[1],
                             valinit=nominal)
        slider.on_changed(getattr(obj, method)) # this may not be right
        self.sliders.append(slider)
        return slider

    def plotme(self, objs):
        ybot = 0.03 * (len(self.sliders) + 3)
        A = plt.axes([0.15, ybot, 0.65, 0.50])
        self.OPs = []
        for obj in objs:
            P = A.scatter(obj.x, obj.y)
            self.OPs.append((obj, P))
        plt.show()

class Data(object):
    def __init__(self, name, x0, y0):
        self.name = name
        self.a = 1.0
        self.p = 1.0
        self.x0, self.y0 = x0, y0
        self.setstuff(a=1.0, p=1.0)
    def setstuff(self, a=None, p=None):
        if a != None:
            self.set_a(a)
        if p != None:
            self.set_p(p)
    def set_a(self, a):
        self.a = a
        self.x = self.a * self.x0
        self.y = self.y0**self.p
        self.xy = np.vstack((self.x, self.y))
    def set_p(self, p):
        self.p = p
        self.x = self.a * self.x0
        self.y = self.y0**self.p
        self.xy = np.vstack((self.x, self.y))