The documentation for LayerContainer#bottom_value states that:
  "If padding is set to :padded, a 15% padding is added below the 
   lowest value.  If the lowest value is greater than zero, then 
   the padding will not cross the zero line, preventing negative 
   values from being introduced into the graph purely due to padding."


Instead, the implementation ignores the argument and inserts a padding, even pushing the bottom_value below zero.