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Context. The distribution of chemical elements in the Galactic disc can reveal fundamental clues on the physical processes that led to the current configuration of our Galaxy. Aims. We aim to map chemical azimuthal variations in the Galactic disc using individual stellar chemical abundances, such as those of calcium and magnesium, and we discuss their possible connection with the spiral arms and other perturbing mechanisms. Methods. Taking advantage of Gaia Data Release 3, we mapped [Ca/Fe] and [Mg/Fe] fluctuations in a region of about 4 kpc around the Sun using different samples of bright giant stars. We implemented a kernel density estimator technique to enhance the chemical inhomogeneities. Results. We observed clear radial gradients and azimuthal fluctuations in the maps of α elements with respect to iron abundances for young (⪅150 Myr) and old (⪆2 Gyr) stellar populations, whose amplitudes depend on the considered chemical species. In the young population, stars within the spiral arms (mostly the Sagittarius-Carina arm and the upper part of the Local arm) are globally more metal-rich and calcium-rich (~0–0.19 dex) but more [Ca/Fe]-poor (~0.06 dex) and [Mg/Fe]-poor (~0.05 dex) than the stars in the inter-arm regions. This indicates higher enhancements in iron than in α elements within the spiral arms. This depletion in [α/Fe] is discussed in the context of different theoretical scenarios, and we compare it quantitatively to a 2D chemical evolution model that accounts for multiple spiral arm patterns. Interestingly, the [Ca/Fe] maps of the old population sample present clear deficiencies along a segment of the Local arm as traced by young populations. We caution that for this old sample, the quality of the obtained maps might be limited along a specific line of sight by the impact of the Gaia scanning law. Conclusions. Our understanding of the chemical evolution of the disc changes from a simplistic 1D radial view to a more complete 2D perspective that combines radial and azimuthal trends and small-scale variations. This study has confirmed the importance of using individual chemical diagnostics as tracers of the spiral arms in disc galaxies. We suggest that the observed α-abundances should be accounted for by models and simulations when the spiral arm lifetimes are addresed. [ABSTRACT FROM AUTHOR]