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While we are working to bring Workbench functionality in line with Mantid Plot, there are a few features are as yet only available in Plot.
Almost all interfaces have been moved over to Workbench, with the exception of the General category.
In Mantid Plot you can right-click on a workspace and show SliceViewer or SpectrumViewer. Within workbench we are combining these (into just the SliceViewer). Some of the functionality of SpectrumViewer within Plot has been implemented in Workbench but not all. Open Spectrum Viewer to see what options there are!
Also a key feature of SliceViewer in MantidPlot is the peaks viewer functionality. It can show you where assigned peaks are in comparison to the raw data. This is being added to Workbench very soon!