Margin calls are mechanisms put in place by your Forex broker in order to keep your used margin secure. Remember, your used margin is allocated by your broker as the collateral for funds borrowed from your broker. A margin call happens when your free margin falls to zero, and all you have left in your trading account is your used, or required margin. When this happens, your broker will automatically close all open positions at current market rates.
In particular I've made the interface for beginning a new backtest a lot simpler by encapsulating a lot of the "boilerplate" code into a new Backtest class. I've also modified the system to be fully workable with multiple currency pairs. In this article I'll describe the new interface and show the usual Moving Average Crossover example on both GBP/USD and EUR/USD.
Foreign exchange (forex) or FX trading involves trading the prices of global currencies, and at City Index it is possible to trade on the prices of a huge range of global currencies. Currency trading allows you to speculate on the movement of one currency against another, and is traded in pairs, for example the Euro against the US Dollar (EUR/USD).
Often, closing one losing position will take the margin level Forex higher than 5%, as it will release the margin of that position, so the total used margin will decrease and consequently the margin level will increase. The system often takes the margin level higher than 5%, by closing the biggest position first. If your other losing positions continue losing and the margin level reaches 5% once more, the system will just close another losing position.

Local Portfolio Handling - In my opinion carrying out a backtest that inflates strategy performance due to unrealistic assumptions is annoying at best and extremely unprofitable at worst! Introducing a local portfolio object that replicates the OANDA calculations means that we can check our internal calculations while carrying out practice trading, which gives us greater confidence when we later use this same portfolio object for backtesting on historical data.