Use combine instead of combineignoringNaN (for compatibility) The ignoring NaN part was not actually necessary but just used for safety. combineIgnoringNaN is availbale only in jfreechart >=1.0.15

SYMBOLSET
SYMBOL
  NAME 'point'
  TYPE ELLIPSE
  POINTS
    1 1
  END
  FILLED TRUE
END
SYMBOL
  NAME "square"
  TYPE VECTOR
  POINTS
    0 0
    0 1
    1 1
    1 0
    0 0
  END
  FILLED TRUE
END
END