Negative values

var neg_data = [

  {"period": "2011-08-12", "a": 100},

  {"period": "2011-03-03", "a": 75},

  {"period": "2010-08-08", "a": 50},

  {"period": "2010-05-10", "a": 25},

  {"period": "2010-03-14", "a": 0},

  {"period": "2010-01-10", "a": -25},

  {"period": "2009-12-10", "a": -50},

  {"period": "2009-10-07", "a": -75},

  {"period": "2009-09-25", "a": -100}

];

Morris.Line({

  element: 'graph',

  data: neg_data,

  xkey: 'period',

  ykeys: ['a'],

  labels: ['Series A'],

  units: '%'

});