matplotlib Code

from matplotlib.pyplot import figure, show
import matplotlib.transforms as transforms
from matplotlib.patches import RegularPolygon
import matplotlib.agg as agg
from numpy import arange, sin, pi
from numpy.random import rand

class ClipWindow:
    def __init__(self, ax, line):
        self.ax = ax
        ax.set_title('drag polygon around to test clipping')
        self.canvas = ax.figure.canvas
        self.line = line
        self.poly = RegularPolygon(
            (200, 200), numVertices=10, radius=100,
            facecolor='yellow', alpha=0.25,
            transform=transforms.IdentityTransform())

        ax.add_patch(self.poly)
        self.canvas.mpl_connect('button_press_event', self.onpress)
        self.canvas.mpl_connect('button_release_event', self.onrelease)
        self.canvas.mpl_connect('motion_notify_event', self.onmove)
        self.x, self.y = None, None

    def onpress(self, event):
        self.x, self.y = event.x, event.y

    def onrelease(self, event):
        self.x, self.y = None, None

    def onmove(self, event):

        if self.x is None: return
        dx = event.x - self.x
        dy = event.y - self.y
        self.x, self.y = event.x, event.y
        x, y = self.poly.xy
        x += dx
        y += dy
        #print self.y, event.y, dy, y
        self.poly.xy = x,y
        self._clip()

    def _clip(self):
        self.line.set_clip_path(self.poly.get_path(), self.poly.get_transform())
        self.canvas.draw_idle()


fig = figure(figsize=(8,8))
ax = fig.add_subplot(111)
t = arange(0.0, 4.0, 0.01)
s = 2*sin(2*pi*8*t)

line, = ax.plot(t, 2*(rand(len(t))-0.5), 'b-')
clipwin = ClipWindow(ax, line)
show()