Is there a diagram showing the requirements for each promotion?

Solution 1:

Well I managed to produce one myself, though it took a few tries until I got the layout right. Click the image to see it full-sized. Explanation and notes below.

Notes:

• On the left are the unit types. An archer signifies all regular ranged units, a cannon all siege units, etc.

• The promotions appear in black rectangles. A promotion with more than one rank lists all the ranks with their effects. Acquiring a higher rank requires the rank before it first.

• The leftmost promotions are the basic promotions - those do not require anything except for themselves. The rest of the promotions are either blue - unlocked from rank 1 of any of the base promotions; yellow - unlocked from rank 2; or red - unlocked from rank 3. For example, unlocking the Sortie promotion requires either Dogfighting rank 2 or Interception rank 2.

• Some of the "melee promotions" are limited to a subset of melee unit types - in those case a small icon signifying the allowed unit types appears above the promotion. For example, Charge is only available for mounted units (horseman / knight / lancer etc.) and tank units (Panzer, Tank, Modern armor, Giant Death Robot). If no icon appears, it means the promotion is available to any unit types appearing on the left. The Rifleman icon is for units without a ranged attack but that use gunpowder technology, such as a Rifleman or an Anti-Tank Gun.

• All images and text copyright of Firaxis. Also, yes, the diagram is missing the woodsman image :(