what to write in figure legend

• The caption for a figure appears below the graphic; for a table, above. It is easy to get this wrong accidentally.
• Typically, boldface or underscore the word “Figure” or “Table” and the associated number in the caption, then present the caption in plain text with only the initial letter of the caption and any proper names in the caption capitalized (see example below).
• Always concentrate on completeness and concreteness as you caption figures and tables. “Figure 3: Air flow” is far less illuminating and accurate than the following:

In papers written for classes and submitted to journals, every table and figure should include a caption, honoring these common practices:

A table of data or graph which is taken from another work should be acknowledged as a reference like any extract from a published source. In the context of a graph, it is numbered and labelled a figure, e.g. Figure 1.18, because it is a visual source displaying numeric data; this is written in a caption underneath the graph. The source citation is placed after the title of the graph as shown below, with the author’s surname, the year of publication and page number (if applicable) in brackets.
For a table, the number and title by which it can be identified are written above the fields of data, and not underneath e.g. ‘Table 2: Analysis of. ‘ The actual source citation in brackets, however, is placed beneath the table.

References:

http://www2.le.ac.uk/offices/ld/all-resources/writing/harvard/content/1.18-table-or-graph
http://libguides.ioe.ac.uk/c.php?g=482485&p=3299866