Hi, Greeeg.
I've not found a way to do this despite many experiments. It does not appear to be designed that way.
You can add text and other annotations to a 2D graph by selecting the desired Object > Insert choice, but you can't place the equations within the space in which the graph is displayed. With 3D graphs, you cannot annotate the graph itself or place the equation(s) within the space occupied by the graph.
To combine the equation(s) and graph in the way you want, you will need to combine them within another document or image-editing application, such as Photoshop.
To use the graphs and equations you've created in Grapher within other documents, such as scientific report or an image-editing application, see the topic "Creating images of graphs and equations" in Grapher Help. In Grapher, select Help > Grapher Help. In the Help Viewer window displaying Grapher Help, click "See all Topics", then select that topic from the list.
Good luck!
😉 Dr. Smoke
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Troubleshooting Mac® OS X