Try to find "default" data items for start and end in period panel This is not using the DefaultValue mechanismn which also exists as the DefaultValue mechanism is deprecated and a candidate for removal.

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