Im rendering graph by morris js. Im using data from mysql database by JSON.

Everything works fine, but I would like to add one more feature to the graph. (change dot color if there is something in buy action).

My JSON:

[{"longdate":"2014-08-20 18:20:01","price":"1620","action":"buy"},{"longdate":"2014-08-20 18:40:01","price":"1640","action":""},{"longdate":"2014-08-20 19:00:01","price":"1620","action":""}]

So I would like to change dot color for values with buy action.

My code for graph:

$.getJSON('results.json', function(day_data) {

Morris.Line({
  element: 'graph',
  data: day_data,
  xkey: 'longdate',
  ykeys: ['price'], 
  labels: ['Cena'],
  lineColors: lineColor,
  pointSize: 0,
  hoverCallback: function(index, options, content) { 
 var date = "<b><font color='black'>Data: "+day_data[index]['longdate']+"</font></b><br>";
     var param1 = "<font color='"+lineColor[0]+"'>Cena - "+day_data[index]['price']+"</font><br>";
     return date+param1;
  },
  xLabelFormat : function (x) {
   return changeDateFormat(x);
  }

  /*My TRIAL
if(action == 'buy'){
pointSize: 4,
lineColors: green,
}
 */ 

});
});

So my code doesnt work, how can I make this working?