I think a useful enhancement which I imagine would take very little work (change graph type) would be as follows:
Since there is an option to select multiple accounts, it currently isn't very useful since they are all plotted independently. If you want to know the results of all the accounts together, it would be nice to have an option to graph them stacked so you can see the overall balance of all the selected accounts. This is useful because it will show the final result of all your asset accounts (like checking) against all your debit accounts (like credit cards).