Grapher: Edit or enter data points directly in the graph with the mouse?
I don't think you can do that but you can put your points in a file and then input the file. I find it much easier to input data using an editor that the boxes on Grapher. Give you file a name that ends in .txt and then you can go to import and after clicking on that the frame that appears has an import button. When clicking on import it opens a Finder window for you to select the file you want.