Trapping broadband electromagnetic radiation over a subwavelength grating, provides new opportunities for hyperspectral light-matter interaction on a nanometer scale. Previous efforts have shown rainbow-trapping is possible on functionally graded structures. Here, we propose groove width as a new gradient parameter for designing rainbow-trapping gratings and define the range of its validity. We articulate the correlation between the width of narrow grooves and the overlap or the coupling of the evanescent surface plasmon fields within the grooves. In the suitable range (≲150 nm), this width parameter becomes as important as other known parameters such as groove depth and materials composition, but tailoring groove widths is remarkably more feasible in practice. Using groove width as a design parameter, we investigate rainbow-trapping gratings and derive an analytical formula by treating each nano-groove as a plasmonic waveguide resonator. These results closely agree with numerical simulations.