Total Yearly Gains/Losses

Visible from both your Dashboard (you'll always see the current year's gain/loss numbers here) and My Taxes pages (titled "Taxable Gains/Losses" on this page, you can toggle between each year's gains/losses), you can see your gains/losses for a given year.

This number is the sum of gains/losses from all of your transactions, throughout any individual year. If you had 1 transaction in 2019 that resulted in a loss of $100, and 1 transaction in 2019 that resulted in a gain of $300, you'll have a gain of $200 for 2019.