>>> """
=======
Buttons
=======

Constructing a simple button GUI to modify a sine wave.

The ``next`` and ``previous`` button widget helps visualize the wave with
new frequencies.
"""
... 
... import numpy as np
... import matplotlib.pyplot as plt
... from matplotlib.widgets import Button
... 
... freqs = np.arange(2, 20, 3)
... 
... fig, ax = plt.subplots()
... plt.subplots_adjust(bottom=0.2)
... t = np.arange(0.0, 1.0, 0.001)
... s = np.sin(2*np.pi*freqs[0]*t)
... l, = plt.plot(t, s, lw=2)
... 
... 
... class Index:
...     ind = 0
... 
...     def next(self, event):
...         self.ind += 1
...         i = self.ind % len(freqs)
...         ydata = np.sin(2*np.pi*freqs[i]*t)
...         l.set_ydata(ydata)
...         plt.draw()
... 
...     def prev(self, event):
...         self.ind -= 1
...         i = self.ind % len(freqs)
...         ydata = np.sin(2*np.pi*freqs[i]*t)
...         l.set_ydata(ydata)
...         plt.draw()
... 
... callback = Index()
... axprev = plt.axes([0.7, 0.05, 0.1, 0.075])
... axnext = plt.axes([0.81, 0.05, 0.1, 0.075])
... bnext = Button(axnext, 'Next')
... bnext.on_clicked(callback.next)
... bprev = Button(axprev, 'Previous')
... bprev.on_clicked(callback.prev)
... 
... plt.show()
... 
... #############################################################################
... #
... # .. admonition:: References
... #
... #    The use of the following functions, methods, classes and modules is shown
... #    in this example:
... #
... #    - `matplotlib.widgets.Button`
...