def legend_data
      if relevant_data?
        retval = []
        layers.each do |layer|
          retval << layer.legend_data
        end
        retval
      else
        nil
      end
    end
def legend_data
      if relevant_data?
        retval = []
        layers.each do |layer|
          retval << layer.legend_data
        end
        retval
      else
        nil
      end
    end
A stacked graph has many data sets.  Return legend information for all of them.