There are several methods of adding a label to a plot. You can use PlotLabel, Labeled, or interactively add text to a plot using Drawing Tools. For a description of adding text to plots using Drawing Tools, see How to: Add Text outside the Plot Area and How to: Add Text to a Graphic.

To add a centered label to the top of a plot, use the PlotLabel option:

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https://wolfram.com/xid/0k2pd80m337laggg15fjlpy-i6c8pl

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You can control the appearance of the label by wrapping it in Style options:

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https://wolfram.com/xid/0k2pd80m337laggg15fjlpy-nbrxa

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If the plot has a background color, the plot extends to include the label:

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https://wolfram.com/xid/0k2pd80m337laggg15fjlpy-fju8i5

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To put a label underneath a plot, like a caption, use Labeled:

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https://wolfram.com/xid/0k2pd80m337laggg15fjlpy-grb52a

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To use the same font and style that is used in graphics, set the label style to "Graphics":

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https://wolfram.com/xid/0k2pd80m337laggg15fjlpy-d048am

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In this case, the background of the plot does not include the label:

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https://wolfram.com/xid/0k2pd80m337laggg15fjlpy-bp6iw

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