Convert dimension values to a dictionary.

        Returns a dictionary of column arrays along each dimension
        of the element.

        Args:
            dimensions: Dimensions to return as columns

        Returns:
            Dictionary of arrays for each dimension

def columns(self, dimensions=None):
        """Convert dimension values to a dictionary.

        Returns a dictionary of column arrays along each dimension
        of the element.

        Args:
            dimensions: Dimensions to return as columns

        Returns:
            Dictionary of arrays for each dimension
        """
        if dimensions is None:
            dimensions = self.kdims
        else:
            dimensions = [self.get_dimension(d, strict=True) for d in dimensions]
        vdims = [d for d in dimensions if d in self.vdims]
        if vdims:
            raise ValueError('%s element does not hold data for value '
                             'dimensions. Could not return data for %s '
                             'dimension(s).' %
                             (type(self).__name__, ', '.join([d.name for d in vdims])))
        return OrderedDict([(d.name, self.dimension_values(d)) for d in dimensions])