How to write and publish a scientific paper - Barbara Gastel, Robert A. Day 2022

Symbols and legends
How to prepare effective graphs
Preparing the tables and figures

If there is space in the graph itself, use it to present the key to the symbols. In the bar graph in Figure 17.1, the shadings of the bars would have been a bit difficult to define in the legend; given as a key, they need no further definition (and any additional typesetting, proofreading, and expense are avoided).

If you must define the symbols in the figure legend, you should use only symbols that are considered standard and are widely available. Perhaps the most standard symbols are open and closed circles, triangles, and squares

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If you have just one curve, use open circles for the reference points; use open triangles for the second, open squares for the third, closed circles for the fourth, and so on. If you need more symbols, you probably have too many curves for one graph, and you should consider putting some of the curves in one graph and the rest in another. Different types of connecting lines (solid, dashed, etc.) can also be used. But do not use different types of connecting lines and different symbols.

As to the legends, they should normally be provided on a separate page, not at the bottom or top of the illustrations themselves. The main reason is that the two portions commonly are processed separately during journal production. Consult the instructions to authors of your target journal regarding this matter and other requirements for graphs.