1. A method performed by a computing system having a processor and a memory, the computing system for determining a performance characteristic of a dataflow graph that includes vertices representing data processing components connected by links representing flows of work elements between the data processing components, the method including: receiving, by the processor, data representing one or more expressions that represent a characteristic of the dataflow graph, defining, in the memory, fields having field types for an in-memory data structure, the defining being based on analysis of the one or more expressions, accessing, by the processor, tracking information produced during execution of the dataflow graph, the tracking information associated with one or more of the data processing components of the dataflow graph, storing, in the in-memory data structure, one or more values determined based on a portion of the tracking information in a corresponding field of the in-memory data structure, wherein the portion of the tracking information is extracted according to the field types for the defined fields for the in-memory data structure, and determining, by the processor, one or more of an amount of data being processed by the dataflow graph and an amount of resources being consumed during the processing by the dataflow graph by: selecting one or more values stored in the fields of the in-memory data structure, and evaluating the one or more expressions by replacing each of one or more variables of the one or more expressions with a corresponding one of the values stored in the fields of the in-memory data structure.