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MantidWorkbench is replacing MantidPlot, while MantidPlot is still fully functional for this release, MantidPlot will not be included in future releases. Please move over to Workbench for all new work, and ongoing work that will last longer than the next few months.
This decision was due to the lack of support for Qt4 and the many major problems this would create for MantidPlot. Also, MantidPlot has not received any updates or many bug fixes for the last year, and as such Workbench is a more stable and functional user interface in many areas.
We have created a high level comparison of the functionality to help you easily see some of the differences.
Workbench is much less cluttered but operates very similarly to MantidPlot, so the experience of moving over to workbench should be smooth and easy. Workbench can even load all of the workspaces (although not the graphs) from a saved MantidPlot Project file if you want to transfer ongoing work.
If you run into any problems or have any questions please ask them on our user forum.