Stratigraphic Plug Alignment for Integrating Plug-Based and XRF Data // Implements the Stratigraphic Plug Alignment (SPA) procedure for integrating sparsely sampled plug-based measurements (e.g., total organic carbon, porosity, mineralogy) with high-resolution X-ray fluorescence (XRF) geochemical data. SPA uses linear interpolation via the base approx() function with constrained extrapolation (rule = 1) to preserve stratigraphic order and avoid estimation beyond observed depths. The method aligns all datasets to a common depth grid, enabling high-resolution multivariate analysis and stratigraphic interpretation of core-based datasets such as those from the Utica and Point Pleasant formations. See R Core Team (2025) https://stat.ethz.ch/R-manual/R-devel/library/stats/html/stats- package.html and Omodolor (2025) http://rave.ohiolink.edu/etdc/view?acc_num=case175262671767524 for methodological background and geological context.