Fills the chart area with the specified background color.
Value type: string, function
var chart = new dhtmlXChart({ view:"radar", fill:"#66cc33", ... })
var chart = new dhtmlXChart({ view:"radar", fill:"#color#", ... })
var chart = new dhtmlXChart({ view:"radar", fill:function(obj){ if (obj.sales > 5) return "#ff9900"; else return "#66cc00"; }, ... })