As answered in .excel.misc --

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I'm not sure how John did it, but you could:

Select the chart
Click in the Formula bar, and type an equal sign
Click on the cell you want to link
Press the Enter key (a text box will appear in the centre of the chart)
With the Textbox still selected, on the Drawing toolbar,
    click the Draw button
Choose Change AutoShape, and select a shape
On the Drawing toolbar, select a line and fill colour
Resize the Autoshape to fit the text
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