FnordMetric Have a try

# code for draw one line
DRAW LINECHART
  XDOMAIN
    FROM_TIMESTAMP(1418635731),
    FROM_TIMESTAMP(1418639331)
  AXIS BOTTOM
  AXIS LEFT TITLE "rxbytes"
  LEGEND TOP left INSIDE;

SELECT
    time AS x,
    value AS y
  FROM
    `rxbytes_eth0`
  WHERE
    time > FROM_TIMESTAMP(1418635731)
    AND time < FROM_TIMESTAMP(1418639331);


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# code for draw antoher line from another data flow
DRAW LINECHART
  AXIS right TITLE "txbytes";

SELECT
    time as x,
    value AS y, "color2" as color
  FROM
    `txbytes_eth0`
  WHERE
    time > FROM_TIMESTAMP(1418635731)
    AND time < FROM_TIMESTAMP(1418639331);