The yield curve is considered inverted when interest rates favor the short-term security because the curve flips and slopes downward instead of sloping upward. This is due to a shorter maturity rate delivering a higher yield. When comparing securities, the spread -- the difference between the longer security's yield and the shorter security's yield -- is a negative number instead of a positive one.
For example, if the long-term security yields 5% and the short-term security 3.5%, the spread is 1.5% because we view it from the long-term security perspective. We plot that point on the corresponding date. Because it and the surrounding figures are all positive, we consider it a great day. When the spread turns negative and stays there, we have an inversion.