field_height (SVG::Graph::Plot)

    # 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