Outlines can be applied to straight lines, curves or any other objects, including text strings.
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To apply an outline colour, select the object and, using the right mouse button, click on a colour in the palette.
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You can also specify other outline attributes using the outline Roll-Up.
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The outline Roll-Up lets you modify the line thickness... arrowheads... line styles... as well as colour. You can preset the options... then select the object and apply.
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The outline Roll-Up makes it easy to update on the fly - change the Roll-Up settings, and apply again.