docs: public StringBuilder appendToBuffer(StringBuilder buf) {
      Iterator<double[]> iter = points.iterator();
      while(iter.hasNext()) {
        double[] data = iter.next();
        for(int i = 0; i < data.length; i++) {
          if(i > 0) {
            buf.append(',');
          }
          buf.append(data[i]);
        }
        if(iter.hasNext()) {
          buf.append(' ');
        }
      }
      return buf;
    }