In the backtest function, display the minimum achieved PNL for the entire tested period.#209

When testing the settings of the signal bot, it would help to display the minimum achieved PNL - especially in loss, so that it is clear whether such a setting can delete the account. At the same time, it would help to enable zooming in the graph during the tested time, because the resulting displayed graph is very distorted and does not show the local extremes that it reached during the tested period.