# File temp/Graph/Plot.rb, line 240
      def field_height
        values = get_y_values
        max = @data.collect{|x| x[:data][Y].max }.max
        if values.length == 1
          dx = values[-1]
        else
          dx = (max - values[-1]).to_f / (values[-1] - values[-2])
        end
        (@graph_height.to_f - font_size*2*top_font) /
          (values.length + dx - top_align)
      end