code: public DataPoint add(DataPoint another) {
          if (type == Type.NONE) {
              _cloneFrom(another);
          } else {
              if (another.type != Type.NONE) {
                  add(another.value());
              }
          }
  
          return this;
      }
docs: Adds value from another data point.
  
  @param another
  @return
  @since 0.3.0