Hi B'
Some Numbers vocabulary:
Spreadsheet: Also called 'Document' or 'Spreadsheet Document': The entire document contained in a Numbers file.
Sheet: a large 'canvas' which can contain one or more Tables, plus other objects such as text boxes, Charts or Images.
Every Document contains at least one Sheet.
Each Sheet has a name, which is displayed on a Tab above the workspace.
The Tab contains nothing except the name of the Sheet to which it is attached (plus a pop-up menu marked v).
The default name of a Sheet is "Sheet n" where "n" is a serial number starting with 1 for the first Sheet.
The names can be changed to give a better indication of what work the Tables on that sheet do.
Table: A set of one or more Cells, arranged in columns and rows.
Each Sheet, when created, contains at least one Table.
Each Table has a Name. The default name of the first table on a Sheet is Table 1. It is recommended that each
Table in a document be given a distinct name, as this makes formulas shorter by removing the need to include
the Sheet name when addressing cells on another Sheet.
Data is placed in Cells in Tables. The Tables are placed on Sheets.
The full address of a cell has three parts, separated by double colons: sheetname::tablename::column and row
Numbers needs only enough of the full address to locate the cell, and will drop the unneeded parts automatically.
Here is an example using the data table you provided, and a second small table to summarize the data and feed it to the pie chart. All three objects, the two tables and the chart, are on the same table. Because the tables have distinct names, the formula used on the summary table will work without change if that table is placed on a separate sheet.

The formula in B2 of Expense Summary is:
SUMIF(Expense log::C,A2,Expense log::B)
"Expense log" is the name of the data table.
The formula is filled down to B3 an B4.
Category names in column A of Expense Summary must exactly match the category names in column C of Expense log.
The chart was created by selecting cells B2-B4 of Expense summary, then clicking the Charts button and choosing the Pie Chart. I made no changes to the chart except to reduce the size to make it fit in the space under the Expense summary table.
Regards,
Barry