Add get_chart method back to Spc

[petebachant]

Present in https://launchpad.net/python-spc/+milestone/0.3:

    def get_chart(self, ax=None):
        """Generate chart using matplotlib."""
        if not mpl_present:
            raise Exception("matplotlib not installed")
        if ax == None:
            ax = pylab
        ax.plot(self._data, "bo-")
        ax.plot([0, len(self._data)], [self.center, self.center], "k-")
        ax.plot([0, len(self._data)], [self.lcl, self.lcl], "k:")
        ax.plot([0, len(self._data)], [self.ucl, self.ucl], "k:")

        if self.violating_points.has_key(RULES_7_ON_ONE_SIDE):
            for i in self.violating_points[RULES_7_ON_ONE_SIDE]:
                ax.plot([i], [self._data[i]], "yo")
        if self.violating_points.has_key(RULES_1_BEYOND_3SIGMA):
            for i in self.violating_points[RULES_1_BEYOND_3SIGMA]:
                ax.plot([i], [self._data[i]], "ro")
        pylab.figtext(0.05, 0.04, "Center = %0.3f" % self.center)
#        pylab.figtext(0.05, 0.01, "StdDev = %0.3f" % self.sd)
        pylab.figtext(0.3, 0.04, "LCL = %0.3f" % self.lcl)
        pylab.figtext(0.3, 0.01, "UCL = %0.3f" % self.ucl)
        #pylab.show()