The UI for aggregates in MongoKitten relies heavily on schemas. It's strongly recommended to read the docs on schema's, in order to effectively use the app.
When building a pipeline, select the necessary stages on the right hand side. To reorder stages, simply drag them by the drag area on the left. Move them up or down in the order you need the stages to appear.
Using the trash icon, you can remove a stage from the hierarchy.
If you want to experiment with an aggregate stage, there are two good solutions to not lose progress.
The first solution is to export the stage in the top bar, and re-import it when you want to roll back.
The second solution is to right-click (macOS) or longpress (iPhone/iPad) and duplicate the stage(s) that you want to change. Now, for each duplicated stage, right-click/longpress to disable the newly created duplicates. These serve as an inline backup, and point of reference.
To preserve a quality experience, we have only implemented the most common aggregate stages. While the other stages are being developed, we're eager to take feedback on any requests for new or existing stages.
Please advise the roadmap for an indication of what's coming up, or provide feedback to accelerate your request.