_mean = function(values) {
        var result = 0;
        for (var i = 0; i < values.length; i += 1) {
            result += values[i];
        }
        return (result / values.length) || 0;
    }
Returns the mean value of the given numbers.
@method _mean
@private
@param {Number[]} values
@return {Number} the mean of given values