By default, Prism the graph on the layout automatically updates when the graph itself updates.
You can choose to place the graph on the layout as an unlinked picture with no link to the original data. You might prefer this option if you are sure the graph is 100% done, and you don’t want to allow any possibility of it being changed. It can also be useful if you want to keep "snapshots" of a graph as a project continues and additional data are collected and graphed.
The choice to make a graph an unlinked picture is on the lower left corner of the Assign Graph to Layout dialog.