Add IRR as an option in the performance graph

The Home Screen performance graph currently allows switching between Return and TTWROR (True Time-Weighted Rate of Return).

TTWROR is useful because it measures portfolio performance independent of deposits and withdrawals. However, for users investing through regular savings plans or periodic contributions, it is also helpful to see IRR (Internal Rate of Return), which reflects the return of the actual invested capital over time.

Since TTWROR and IRR answer different questions (strategy performance vs. return on invested capital), it would be very useful to have IRR available as an additional option in the performance graph.