Thanks for all the comments guys
I am going to modify the diagram with any changes that needed be made. Below are some notes on it:
1. I have not included any UU because it would clutter the diagram.
2. If a line joins another line with an arrow at the end it means that both units upgrade to the same unit. For instance both Longbow Man and Musketman upgrade to Rifleman.
3. I did not include some units as they did not upgrade to anything or had nothing upgrade to them such as Marines. This was more of an oversight than anything as I also have Carriers and Battleships which are not in any upgrade paths.
I will add Marines and any other regular units in for the sake of completeness. It is nice to see all the units that make up say gunpowder or melee units.
If anyone has any ideas or improvments please let me know.