Correlation plot between two standardized molecular lines. The color indicates the Mahalanobis distance, which accounts for the correlation between the two variables. The red ellipse represents a Mahalanobis distance of 1. These plots correspond to the maps shown in Figure 1 (left), Figure 2, and Figure 3 of H. Okubo et al. (2025). In addition, the two panels shown in the upper right are enlarged views of the ¹³CO(J = 1–0)–C¹⁸O(J = 1–0) and ¹³CO(J = 1–0)–N₂H⁺(J = 1–0) plots.