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We present a theoretical and experimental investigation of slow drainage in porous media with a gradient in the grains size (and hence in the typical pores' throats), in an external gravitational field. We mathematically show that such structural gradient and external force have a similar effect on the obtained drainage patterns, when they stabilise the invasion front. With the help of a newly introduced experimental set-up, based on the 3D-print of transparent porous matrices, we illustrate this equivalence, and extend it to the case where the front is unstable. We also present some invasion-percolation simulations of the same phenomena, which are inline with our theoretical and experimental results. In particular, we show that the width of stable drainage fronts mainly scales with the spatial gradient of the average pore invasion threshold and with the local distribution of this (disordered) threshold. The scaling exponent results from percolation theory and is -0.57 for 2D systems. Overall, we propose a unifying theory for the up-scaling of dual fluid flows in most classical scenarii.