Question: I would like to display a symbol label on two lines in the legend for a layer that is symbolized using unique values. I would like both lines of the label to be centered horizontally with the label.
Answer: To make legend labels display on two lines, you need to add a description to the legend items. This is done by right clicking in the lend item on the Symbology tab of the Layer Properties dialog. Select Edit Description… and here you can type in the label name (or paste copied text) and then add a carriage return (by clicking Control + Enter) to split the label so it shows on two lines.
To have the description show up in the legend instead of the label, in the legend properties, on the Items tab, right click the name of the layer in the Legend Items pane and click Properties. Here there are a number of options that relate to labels and descriptions, but at the least on the General tab you can choose to show the descriptions and not the labels for the legend items.
You can find some related documentation here:
The first article describes how to create a description and the second article describes how to display the description in a legend.
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