A comparison line feature would be a very useful feature to have on the portfolio chart/graph shown on the app.
This would allow users to compare their performance with an index (e.g. S&P 500 or FTSE 100) or another stock/ETF.
Monitoring portfolio performance compared to an index will show the user how their investing/trading methods and stock picks are affecting their performance compared to the market.
It would also allow the user to monitor the volatility of their portfolio compared to an index or ETF, and may encourage them to make changes to their portfolio to better match their risk appetite.