$rrd->graph(
  #image          => '-',
  image => "../images/" . $sensor . ".png",
  vertical_label => 'Sensors',
  width          => $width,
  height         => $height,
  start          => time() - $period,
  draw           => { file => "../data/sensor1.rrd", legend => "Source 1", thickness => 2, color => '00FF00' },
  draw           => $draw[0],
  draw           => $draw[1],
  ... and so on ...

);