I subcontracted with a lead developer to administer an inherited, outsourced HighCharts.js app displaying years of stock market data.
The code, split across a dozen .js files, needed frequent debugging due to questionable architectural decisions (the administrator is given monthly data updates in the form of a multi-gigabyte CSV file).
I built a new feature into the app, allowing financial data to be sorted by business quarter in addition to years and months.