Here's a response from the scientist at CPC responsible for this figure. This plot is not in the blog post, only on the CPC web site.

The x-axis in the plot shows the anomalous Warm Water Volume (WWV), which measures the total volume of water warmer than 20C. When the depth of the 20C isotherm (D20) is deeper than normal, more warm water resides above D20, and therefore WWV is above-normal with positive anomaly. A shallower D20 indicates WWV is below-normal with negative anomaly. A positive (negative) WWV anomaly is often a precursor for the development of El Nino (La Nina). The plot is used to show the relationship between WWV and NINO3.4 anomaly during the ENSO cycle. For ENSO events before 2000, there is usually a cyclonic progression of WWV and NINO3.4 anomaly in the phase space. However, after 2000 there is a less systematic progression in the phase space and the amplitude of WWV and NINO3.4 anomaly is much weaker. However, during the development of the current El Nino, we see a clear cyclonic progression in the phase space. For example, from January to March 2015, a rapidly increasing WWV accompanied a slowly growing NINO3.4. While from March to June 2015, a slowly growing WWV accompanied a rapidly increasing NINO3.4.