The Category Breakdown chart is in the Overview tab of Analytics. It shows how email volume is split across your alias categories for the selected time range.
What it shows
Each category you've created (Shopping, Work, Social, etc.) appears as a segment with a percentage of total email volume. Aliases not assigned to any category are grouped under an uncategorised segment.
What it's useful for
It gives you a sense of which parts of your online life are generating the most email. A large Shopping segment might prompt you to audit those aliases. A small Social segment might mean those aliases aren't being used much.
Uncategorised
A large uncategorised segment usually means you haven't assigned categories to most of your aliases. This isn't a problem — but if you want the breakdown to be meaningful, it's worth going through Aliases and assigning categories. See Alias categories.
No categories yet?
If you haven't created any categories, the breakdown will show everything as uncategorised. Create categories from Aliases → Categories and start assigning them to aliases to get useful segmentation in this chart.