Source-plane metallicity radial gradient measurement of one highly anisotropically magnified galaxy (MACS 0717–ID 01131) in our sample. Its image-plane metallicity radial gradient measurement is presented in Figure B1. Top, from left to right: the source-plane reconstructed 2D maps of ﹩{H}_{160}﹩-band surface brightness, stellar surface density (﹩{{\rm{\Sigma }}}_{* }﹩), and surface brightness of emission lines ﹩[{\rm{O}}\,{\rm\small{III}}]﹩, Hβ, and ﹩[{\rm{O}}\,{\rm\small{II}}]﹩. These 2D maps are arranged on the same spatial scale with a scale bar of 1 kpc shown in the leftmost panel. The black contours mark the deprojected galactocentric radii with 1 kpc intervals, with galaxy inclination taken into account. Note that the black radius contours in Figure B1 are obtained from relensing the contours shown in this figure to the image plane of this galaxy. Bottom: metallicity map and radial gradient determination for this galaxy in its source plane. We again use Voronoi tessellation to divide its source-plane reconstructed spatial extent into bins with a constant S/N of 5 on ﹩[{\rm{O}}\,{\rm\small{III}}]﹩, the same as used in the gradient measurement in the image plane. In the right panel, the metallicity measurements in these source-plane Voronoi bins are plotted as magenta points. The dashed magenta line denotes the linear regression, with the corresponding slope shown at the bottom. The metallicity gradient measured in the source plane is ﹩{\rm{\Delta }}\mathrm{log}({\rm{O}}/{\rm{H}})/{\rm{\Delta }}r=-0.031\pm 0.023\,[\mathrm{dex}\,{\mathrm{kpc}}^{-1}]﹩, in agreement with the gradient measured in the image plane, i.e., Δ log(O/H) / Δr = −0.055 ± 0.028 [dex kpc⁻¹].