Currently reporting by tag is only available in the desktop service.
You can find out how much you spent by tag in the My Transactions screen or alternatively you can use the Spending Reports feature accessible in the Manage my Spending dashboard.
Using the My Transactions option gives you all the transactions that you have used this tag for. Using the Spending Reports option shows you the data in category groups for an overall picture of how the money was spent.
Note: Transfers (moving money between your accounts) are not included in your spending reports, and they are included in your transaction history. If there is a discrepancy in amounts, this will be why.
My transactions option:
Go into My Transactions. On the right-hand side, use the Date filter to customise your dates and type the tag you wish to report on in the Tag filter.
All the transactions with that tag will display, and at the top of the screen under the Overview you will see the total debits and credits for that tag for that time period.
Spending Reports option:
Go to the Manage My Spending dashboard and select View Details from the Spending Reports section of the dashboard.
On the right hand-side of the Spending Reports screen use the Date filter to customise your dates and type the tag you wish to report on in the Tag filter.
All data for that tag will display broken down into categories for you as well as represented with graphs. The total figures will be at the top of the screen.