make a cheap plot of the GeoStruct

        Parameters
        ----------
        **kwargs : (dict)
            keyword arguments to use for plotting.

        Returns
        -------
        ax : matplotlib.pyplot.axis
            the axis with the GeoStruct plot

        Note
        ----
        optional arguments include "ax" (an existing axis),
        "individuals" (plot each variogram on a separate axis),
        "legend" (add a legend to the plot(s)).  All other kwargs
        are passed to matplotlib.pyplot.plot()

def plot(self,**kwargs):
        """ make a cheap plot of the GeoStruct

        Parameters
        ----------
        **kwargs : (dict)
            keyword arguments to use for plotting.

        Returns
        -------
        ax : matplotlib.pyplot.axis
            the axis with the GeoStruct plot

        Note
        ----
        optional arguments include "ax" (an existing axis),
        "individuals" (plot each variogram on a separate axis),
        "legend" (add a legend to the plot(s)).  All other kwargs
        are passed to matplotlib.pyplot.plot()

        """
        #
        if "ax" in kwargs:
            ax = kwargs.pop("ax")
        else:
            try:
                import matplotlib.pyplot as plt
            except Exception as e:
                raise Exception("error importing matplotlib: {0}".format(str(e)))

            ax = plt.subplot(111)
        legend = kwargs.pop("legend",False)
        individuals = kwargs.pop("individuals",False)
        xmx = max([v.a*3.0 for v in self.variograms])
        x = np.linspace(0,xmx,100)
        y = np.zeros_like(x)
        for v in self.variograms:
            yv = v.inv_h(x)
            if individuals:
                ax.plot(x,yv,label=v.name,**kwargs)
            y += yv
        y += self.nugget
        ax.plot(x,y,label=self.name,**kwargs)
        if legend:
            ax.legend()
        ax.set_xlabel("distance")
        ax.set_ylabel("$\gamma$")
        return ax