No figured-bass notation representation exists in either Gradus data model (model gap)

Figured bass is a first-class notation element: stacked arabic numerals (with accidentals) below a bass note indicating the harmony above it, positioned at the bass note's x-coordinate, read top-to-bottom in a fixed convention.

Gould Figured bass; Read (full grammar of standard figures and abbreviations); Ross.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.