I'm not able to view your link but I imagine I know what you're referencing. You have to define your prompt as a Presentation Variable (ex. PV_CURRENCY). When you are editing your graph's title, you will then reference the Presentation Variable in the Title (ex. Graph of Spending in @{PV_CURRENCY})

I can't see your link, either.
You can use a variable prompt, perhaps with radio buttons for USD or EUR. The choice will be a pres variable, say P_CURR.

In your analysis, include a case statement column like CASE '@{P_CURR}{USD}' when 'USD' then '$' when 'EUR' then '€' , etc.

Use a formatted narrative view for your title that references that column's position.

In the narrative view, set the "number of rows to return" as 1.