A method executes at a computing device that includes a display. The device displays a user interface that includes a data flow pane and a calculation pane, the data flow pane including a node/link diagram for a data prep flow. A user selects a node in the diagram, and the device populates affordances in the calculation pane according to data fields of a data set associated with the selected node. A first user input specifies grouping on a first data field, and a second user input specifies an aggregation function on a second data field. In response to the user inputs, for each distinct value of the first data field, the device aggregates corresponding values of the second data field according to the aggregation function. Calculated data values are displayed in the calculation pane. The device saves rows of data displayed in the calculation pane as a new data source.