A plugin that charts any account or expense category over time. This plugin would basically be similar to a mixture of the "Show me a chart of my net worth over the past ..." report and the "REPORT_DESCRIPTION_INCOME_EXPENSES_BY_SELECTED_CATEGORY" report plugin. This plugin would allow a user to see how expenditures for utilities, for example, change from month to month or week to week. It might include monthly subtotals for the selected categories as well.
I would suggest that categories include: each of the accounts, net worth, each of the expenses, debt, assets, etc. The user could check boxes for multiple categories (like for the income / expenses report plugin). After selecting one set of radio buttons for time period and another for time increment (day, week, month, quarter, year, etc.), a line graph of the categories over time would be generated and displayed in the web browser.