What is the difference between a legend and a caption? [closed]

I have a graph with boxes of different colours and there is a part that explains what the coloured boxes mean.

Is this my graph caption or its legend? Are those words synonyms or used in a particular context?

The term legend is used to describe information in charts, for example, to identify time series.

The term caption is used to identify or describe an entire chart or some other graphical content or table.

Both terms are, effectively, titles.

A legend provides a mapping that tells you what are being represented by the symbols in a graphic, usually a chart.

A caption is some text accompanying a graphic, for example a chart. It is not part of the graphic. It just provides some textual description of the graphic.

"Captions" are titles. They are usually placed below illustrations. They are never complete sentences (except that when the caption and the legend run together, the caption ends with a full stop).

"Legends" are explanations and can take one or more sentences.

Here are the examples:

Caption: Mars Bridge in 1909

Legend: This photograph depicts the topography of Mainland as it looked like in 1909.