Temperature-salinity diagrams (TS-plots) are used in physical oceanography to identify water masses. Since water masses also have distinct oxygen concentrations and exist along particular isolines of potential density (isopycnals), it is helpful to combine all four parameters into one plot for the purpose of water mass identification.
This function allows you to create TS-plots with isopycnals (using potential density) and dissolved oxygen concentration as the z-axis. This is an adaptation of the ggTS function created by David Kaiser (http://doi.org/10.5281/zenodo.3901308).