function stackPos (a, b) {
      stackOffsetPos = stack.negative[a] || 0;
      stackOffsetNeg = stack.positive[a] || 0;
      if (b > 0) {
        stack.positive[a] = stackOffsetPos + b;
        return stackOffsetPos + b;
      } else {
        stack.negative[a] = stackOffsetNeg + b;
        return stackOffsetNeg + b;
      }
    }
Formats the marker labels.
@param {Object} obj - Marker value Object {x:..,y:..}
@return {String} Formatted marker string