A computer system transforms data. A first user builds a flow diagram. Each node in the flow diagram specifies an operation: to retrieve data, to transform data, or to create an output dataset. The flow diagram includes a subgraph having a plurality of nodes. When the first user initiates execution, the system performs the operations in the subgraph sequentially as specified. When the first user requests the system to save the flow diagram, the system stores the subgraph. The system displays a user interface and the subgraph in the user interface. A second user requests the system to build another flow diagram based on the subgraph. In response, the system retrieves the subgraph from memory. When the second user requests the system to execute the flow diagram built by the second user, the system performs the operations of the nodes in the subgraph sequentially as specified in the subgraph.