I'm going to take the “Expense Piechart” report in GnuCash 2.6.7 as my primary example; other reports may have slightly different options.
- Open the report you want to use.
- Click “Options" in the toolbar or “Report Options” in the Edit menu.
- In the “Accounts” tab, the “Show Accounts until level” option (or “Account Display Depth” in some other reports) cuts off subaccounts, and you may wish to increase it. For example, I keep expense accounts like “Expenses:Entertainment:Games”, and if I want to see that specific account I have to set this option to 3, instead of the default 2.
- In the Display tab, the “Maximum Slices” (“Maximum Bars” for bar charts) controls how many accounts are shown before the rest are grouped as an “Other” entry. However, it only goes up to 24.
If you want to keep this configuration for future use, you can either leave the report tab open indefinitely, or save a custom report using the “Save Report Configuration As…” command in the toolbar or Reports menu.
Since you asked about expenses in particular: if you are looking for a complete list with more than 24 items, you can use the Income & Expense → Cash Flow report. This report will list out every expense account (and income account, separately) with nonzero transactions. However, it does not have a pie at all and does not display percentages.