In a graph, the legend is used to provide information about the data series, such as the name, color, or pattern corresponding to each data set. It helps in identifying and understanding the representation of different data series in the graph. Other options, such as:
Data point: Represents an individual value within the data series.
Data table: Displays the raw data used for creating the graph.
Plot point: Not a standard term used in graphing.
Thus, the correct answer is \( \mathbf{(D)} \) legend.