convert_to_png

# File lib/rdoc/diagram.rb, line 281
    def convert_to_png(file_base, graph)
      str = graph.to_s
      return @diagram_cache[str] if @diagram_cache[str]
      op_type = @options.image_format
      dotfile = File.join(DOT_PATH, file_base)
      src = dotfile + ".dot"
      dot = dotfile + "." + op_type

      unless @options.quiet
        $stderr.print "."
        $stderr.flush
      end

      File.open(src, 'w+' ) do |f|
        f << str << "\n"
      end

      system "dot", "-T#{op_type}", src, "-o", dot

      # Now construct the imagemap wrapper around
      # that png

      ret = wrap_in_image_map(src, dot)
      @diagram_cache[str] = ret
      return ret
    end