The Sales by Country plugin sums all order totals together, regardless of the currency in which each order was placed. That is, if you have USD as the base currency and three orders for 100 USD, 100 EUR and 100 GBP, the plugin calculates a total of sales for of 300 USD.
The plugin does not implement filters that allow to change its behaviour, therefore it must be modified directly. To do so, search the following text in the plugin files:
AND order_item_meta.meta_key = '_line_total'
and change it to this:
AND order_item_meta.meta_key = '_line_total_base_currency'
This modification will make sure that the query will pick up the order total in base currency, which has been saved when the order was generated, and return the correct result.