The PL by Category Dashboard flips the rows and columns.
Go to the green dropdown on the Dashboard, and select PL by Category. This has the effect of making your columns into rows, and your rows into columns, so now you'll see your Departments on the left, and your categories across the top.
You'll also see a category menu above the table on the left that allows you to choose which categories to view.
If you're looking at your whole chart of accounts, this view may be difficult to manage, and will load a little more slowly, so there is an option in Settings > Preferences which you can set to just show the group totals, and not the individual categories.