Like any writing, you'll need to follow an accepted style when preparing your Illustrations. Check the publication or the organization's style manual for guidance on preparing the Illustrations.
Many publications follow a standard style manual like The Chicago Manual of Style, CBE Style Manual, or the Publication Manual of the American Psychological Association. Each one specifies details for illustrations, both tables and figures.
Check carefully the Illustrations in any journal, magazine, Web site, or organization for specific style issues. For example, many styles call for the titles to go above tables and below figures, but some organizations place all titles above both tables and figures. Look carefully at extended discussions of tables and figures in this module on Illustrations.
For class assignments, ask your instructors what publication style they would like you to follow.