Make the two dimensional histogram

        Parameters
        ----------
        da: xarray.DataArray
            The data source

def hist2d(self, da, **kwargs):
        """Make the two dimensional histogram

        Parameters
        ----------
        da: xarray.DataArray
            The data source"""
        if self.value is None or self.value == 'counts':
            normed = False
        else:
            normed = True
        y = da.values
        x = da.coords[da.dims[0]].values
        counts, xedges, yedges = np.histogram2d(
            x, y, normed=normed, **kwargs)
        if self.value == 'counts':
            counts = counts / counts.sum().astype(float)
        return counts, xedges, yedges