code: public void addPoint(double x, double y) {
          numItems++;
          double xMinusMeanX = x - meanx;
          double yMinusMeanY = y - meany;
          sumsq += xMinusMeanX * yMinusMeanY * (numItems - 1) / numItems;
          meanx += xMinusMeanX / numItems;
          meany += yMinusMeanY / numItems;
      }
docs: Update the covariance with another point.
  
  @param x
  x value
  @param y
  y value