Final answer:
A data flow diagram (DFD) is a useful tool for visualizing the information flow and relationship between input and output documents in system analysis, and a line graph can effectively illustrate changing trends or patterns between two variables.
Step-by-step explanation:
A data flow diagram (DFD) is a useful tool for system analysis. It shows the relationship between input and output documents. By visualizing the flow of information, a DFD helps in understanding the system and how it operates, and can be especially useful in the design and analysis phases of the system development life cycle.
In terms of data representation, a line graph can often be the most effective format for illustrating a relationship between two variables that are both changing, such as showing trends or patterns over time.