A central goal of Augmented Reality (AR)-based Situated Visualization (SV) is to seamlessly integrate digital information into its relevant physical context. While existing frameworks describe numerous design dimensions, the field lacks a rigorous model to evaluate this integration. To address this, we introduce SCORE, a framework for quantifying diegesis - a narratology concept describing the extent to which an element belongs to its narrative world. Grounded in a systematic analysis of 50 contemporary SV works, SCORE defines five dimensions of diegesis in SV: Spatial proximity, Concreteness, cOherence, Referential context, and Environmental context. In addition to qualitative comparison, our framework also provides a quantitative measure of diegesis, enabling SCORE to distinguish AR-based SVs that prior models have treated as theoretically equivalent. We validate the framework by demonstrating a consistent correlation between higher scores and positive usability outcomes. Based on these findings, we offer insights for SV designers.