There should be a Menu Item within the Market to view the supply and demand of goods across multiple ports. The menu item would open and show a list of all available goods (Cotton, Iron, Rum, Tools, etc.). Clicking the good will open a supply/demand chart for the good showing the ports the good is sold from and purchase price, and all ports that demand that good and its selling price. The supply/demand chart should be limited, showing costs for only ports that you have warehouses in. This would make it easier to build and update trade routes.

I have a few alternative suggestions and add-ons for this that I may post later.

There is such a chart. If you click the image of the anchor chain in the top left corner of your screen (left side of header) you will be taken to the active ports list. This will also only show ports you have a warehouse in.

Well there it is. Not the most obvious link. Should have a better link name. I would have never clicked on it.

It is also located on the market page. Right hand menu as "view active ports". I just tend to use the header link for convenience. You can also access fleet and ships pages using the right side of the header.

I saw that as well after the fact. Still should have a better name.