it describes the data shown simply in a short phrase
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You find the main idea of the graph then make the title based on that.
The title of a graph should be an indication of the information that the graph is meant to convey. It may not be a direct description of the data but it should certainly be relevant to the data being used. For example, a graph showing number of residents against years might have a title of "Growth in the town". The title refers not to the data but to the information the graph is showing.
The first step in reading a graph is to look at the title to learn what the graph is about.
Title
to know what is the thing that you are expiramenting