The easiest way to make tree diagrams is to use a family history program. Don't you have one on your computer? I think they will all do the job you want with many variations! Click on the Genealogy Software link at the top of this page to find out about the programs available.
On the other hand there is a program called TreeDraw which is just for constructing a tree. You can do it from scratch or import a GEDCOM file into it.
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