Auto-Align

What it does

• It keeps what is lined up, lined up. Shapes whose centres or edges sit within a few pixels of each other snap onto one shared grid line, so a column of steps of different widths stays centred and the arrows down it stay dead straight. Two shapes joined by a straight arrow line up from a little further apart.
• It keeps even spacing even. Rows that were an equal step apart, like the rows of a key, stay an equal step apart.
• It never moves anything far: a shape moves at most the few pixels to the line it already nearly shares, so a shape that was roughly in place stays roughly in place, only neater.
• It never makes things worse. A snap that would make two shapes overlap, or nudge a shape part-way out of the lane or frame it sits in, is skipped for both.
• It works on every element on the active tab, not just a selection.
• Arrows follow their shapes. An arrow attached to a shape stays attached as the shape snaps; only loose arrow ends snap on their own.
• Locked elements stay put. A locked element, or anything on a locked layer, is not snapped.
• Shapes keep their proportions. A circle, diamond or actor drawn square stays square; one drawn wide stays wide, so a flowchart's decision diamonds keep their shape. An element whose aspect ratio you locked keeps its ratio.
• Running it again changes nothing. Once a tab is tidy, the Tidy Up panel shows Auto-align greyed out with "Already tidy".

When to use it instead of Auto Layout

Reach for Auto-Align when the diagram already looks right and you just want it crisp: rows that should share a baseline, columns that should share an edge, shapes that should be the same size. If instead the diagram has no real layout, or you want positions computed from how shapes are connected, use Auto Layout instead. Auto Layout already lands every shape's centre on the grid, so its result needs no Auto-Align afterwards.