Showing overpayments in a graph

One of our helpful users pointed out that, when a tenant overpays, the system was showing a negative outstanding. This is probably confusing, it certainly looks odd on the graph. Instead, when that happens we show no outstanding, reduce the amount received to show the amount that should have been paid, and then show the outstanding separately. Here's what we mean - a £200 a month tenancy, with a £380 payment on last month. Perhaps the tenant was paying this month's early?