What are the X and Y axes on the graph?

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The relationship shown is between gas compositions, so the axis on the horizontal is the oxygen content by volume, and the axis on the vertical is the hydrocarbon content by volume. In gas mixtures, percent by volume is a standard way to express concentration and is effectively the mole fraction, so tracking these two components makes sense when you want to see how changing oxygen levels affects hydrocarbon concentration. Read the hydrocarbon percentage by locating it on the vertical axis at the oxygen percentage you see on the horizontal axis. This setup is useful in tanker operations for assessing atmosphere safety and flammability risk, where oxygen level and hydrocarbon content together determine how dangerous the mixture is.