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The ability to extract chromatograms from UV spectral data was introduced in OpenLAB CDS 2.2. You can find the relevant settings in the Processing Method under Extraction -> Chromatogram in the UV tab (in older versions of OpenLAB CDS there was no UV tab in this section and you could only extract chromatograms from MS data) as shown below:
Once you have the extraction parameters set in the processing method linked to your data, save the method and reprocess. The act of reprocessing will extract the chromatograms as defined in the method.