The following is taken from Guidelines and Standards for Tactile Graphics, 2011.
5.6.4 Occasionally, information contained in a note or notes is essential for an understanding of the graphic before starting to read it. In this case, insert a transcriber's note before beginning the graphic.
Example: "Note(s) shown in the graphic below."
Place the note on the next line, preceded by the appropriate reference indicator starting in cell 7, with runovers in cell 5. When more than one note is shown, no blank lines should be left between the notes.