Philip,
Interesting idea.
The result you experienced is due to your algorithm logic being placed in your handle_data() function.
Because you are using minute data, this function is called every minute, and your algorithm gains a bigger position than your portfolio has cash. This can be avoided by issuing orders only if there is not currently an open order.
I've updated the algorithm to not trade if there is currently an open order.
Here's the updated backtest. As you can see, the result is much more reasonable.
Best,
Lota
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