The "value at transaction" is calculated as the sum of transactions with their converted historical value at the time of the transaction.
The individual values can be viewed on the "Trade Prices" page.

The values can be negative if the value of all incoming/purchased coins is lower then the value of all outgoing/sold coins.

An example:
- You bought 1 BTC for 1000 USD/BTC.
- Then you sold 0.5 BTC for 4000 USD/BTC (value = 2000 USD)
- As a result, you get 0.5 BTC, value at transaction = -1000 USD (since 1000 - 2000 = -1000)