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Large mineral grains that have undergone shock metamorphism or tectonic stress can exhibit plastic deformation. Using in situ micro X-Ray diffraction, such deformation is revealed in a 2D detector image by distortion of diffraction spots into an angular spread or ‘streaking’ along Debye ring or chi (χ) dimension; this can provide a quantitative measurement of plastic deformation via full width at half maximum (FWHMχ) measurement of the diffraction profile along χ. Often the X-ray intensity is not a single homogeneous streak, but is partitioned into discrete orientationally-related spots along the Deybe ring, called asterism. These complex peaks require more sophisticated fitting. This study introduces a user-friendly, high efficiency data fitting program, Best Fit for Complex Peaks (BFCP), executed within Matlab®. Using XRD data as examples, we fit the integrated complex peaks by a Pseudo-Voigt function and assess …