Astah has several alignment features to align UML model elements nicely with easy step.
Example) Feature to align model elements vertically
But still sometimes you may want to move model elements individually just to place them in a right place. In that case, hold [Command] key down when you move them, so that they will move 1 pixel at a time.
Also you can show Grid on the diagram from [Tool] – [System Properties] setting. See detail (TIPS: Show Grid in diagram)
Cheers,
SJ
