Top: Set of mass-fraction CH₄ profiles, demonstrating the linear spline parameterisations. The black dashed line shows the equilibrium profile for CH₄ for [M/H]=0 and C/O = 0.55, for the temperature profile illustrated by the grey dotted line. Bottom: Equilibrium chemistry models have large variations in abundances throughout the atmosphere. In this model, we fixed all of the abundances to the median retrieved values of Table 3, except for that of methane. The different colours correspond to the same abundance profiles as in the top panel. Even though these parameterisations only affect the methane abundance, the impact on the spectrum is clearly distinct from the variations caused by the hotspot.