Build a toggle/slicer for currency conversion

// Keep XRates disconnected. Assuming that
// the table stores rates of conversion from
// the indicated currency to some other, but common,
// currency. In other words, the rate tells us
// the amount of the currency that you have to pay for
// 1 unit of the target currency (whatever it is,
// but for this calculation it does not matter in the
// slightest).
//
// Here's a measure that will display totals
// in any currency you want...

[Total] =
IF( HASONEFILTER( XRates[Currency] ),

	var __targetCurr = SELECTEDVALUE( XRates[Currency] )
	var __targetCurrXrate = SELECTEDVALUE( XRates[xrate] )
	return
	SUMX(
		// Iterating over currencies in the current
		// context to speed up the calculation. If
		// we iterate over products there'll be much
		// more computation. There are far fewer currencies
		// than products, right?
		VALUES( 'Product'[Currency] ),
		
		// get the currency of the
		// products we right now iterate over
		var __originalCurr = Product[Currency]
		// got the xrate for this currency
		var __originalCurrXrate =
			CALCULATE(
				SELECTEDVALUE( XRates[xrate] ),
				XRates[Currency] = __originalCurr,
				ALL( XRates )
			)
		// get the total price of the products
		// with the current currency
		var __total = CALCULATE( SUM( 'Product'[Price] ) )
		// express the total in the original xrate
		var __valueInCommonCurr = 
			__total * __originalCurrXrate
		// express the above total in the target currency
		var __valueInTargetCurr =
			__valueInCommonCurr / __targetCurrXrate
		return
			__valueInTargetCurr
	)
)