Hi Katrina...I've contacted the BANA TG Guidelines authors and this is our combined response:
The example of the Graphics Symbols Page (GSP) that is shown in the Guidelines is taken from a tactile graphics supplementary volume of a physical geography book that has both maps and graphs. One of the points that this GSP was meant to show was how the symbols may be categorised: “On maps” and “On graphs”.
In a Mathematics textbooks, we do not include the symbols for the components of Cartesian coordinate graphs such as grid lines, axis lines, point symbols, plotted lines, etc. because it limits their use on other graphics throughout the volume. For example if we defined a certain line as the axis line, we would not be able to use that line for anything else within the volume. Also it is not really valuable content to simply define a point as a point. or a line as a primary line. We have many examples in the guidelines where lines are included in a drawings but not identified in a key for the braille reader. (See Unit 5, the Patterns example--the dashed lines are not labeled or identified in a key. Unit 6, section 220.127.116.11 bar graphs, the "Pay for Animal Actors" graph does not include a key for the axis lines and lead lines. The GSP example is not a Mathematics specific example, and was not intended to recommend that mathematic diagrams symbols and lines should to be listed. It should also be noted that the intent of the GSP, unlike the Special Symbols Page, that shows “unusual” symbols, is to show commonly used symbols (rivers, mountains, water). This eliminates the need to repeat them throughout the volume as well as ensures that graphics symbols used for the same item are always the same.
A simple answer to a complex question is that a key must be included for the reader when something is used within a graphic that needs to be identified for a student. Unless the reader needed to be able to identify a certain area on the graphic as the "grid lines" the grid lines do not need to be identified in a key.
Use of keys to include information you listed in your email is a misinterpretation of the intent of the GSP. The Tactile Graphic Supplement that accompanies the Guidelines and Standards for Tactile Graphics shows many example of the proper use of keys. If you have any further questions, please let me know.