You can add comments directly on the graph, using the contextual menu (right-click, or space bar, or Option-Click)
Note you can have a different set of comments for linear scale / log scale, calendar days / normal days and every periods.
You can, as well, simply click on a comment and move it. If you click while holding the shift key, the comment will be copied.
When you select “Add Comment”, a data entry panel opens. You can enter your text, pick the format and the color, using the “Fonts” and “Colors” buttons.
When you position the mouse cursor over a comment, a right click (or Option-click) display a context menu to delete, edit, copy or move the comment.