Use PriceOffset, as discussed here:
https://opserver.de/ubb7/ubbthreads.php?ubb=showflat&Number=486155#Post486155
Quote
PriceOffset
Amount added to historical or live prices of the current asset, for special purposes such as shifting negative prices to the positive scale. Set it after selecting the asset, but before loading history for affecting historical prices.

https://zorro-project.com/manual/en/spread.htm

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