Each portfolio should have an associated "share" price based on an arbitrary NAV for each "share" when portfolio is created. Adding or removing cash from portfolio then simply buys/sells portfolio "shares". This means that portfolio performance does not change (charts especially) when you add/remove money. %up/down is based on portfolio "share" price inc/dec from initial NAV at portfolio setup time.

Just like ETFs measure performance.

This would also means you should add a "performance view" to the portfolio chart for the following:

YTD
Since inception
Since any manually input start date and end date.

This was used by the Motley Fool back in the 90s. They had a spreadsheet portfolio manager that illustrated the principle.