Revolut set users FX rate at the time when they make a transaction so it makes slightly more sense to include the graphs, to enable users to see why their bottle of water cost more when they bought it in the morning vs the second bottle that they bought in the evening.
Their CEO used to be a currency trader too (& apparently still enjoys a bit of speculation), which might have something to do with the decision to include them
Monzo uses the MasterCard rate which is updated once each day on their website so there isn’t much to put in a graph. And since the rate is applied when the transaction settles, rather than when it’s authorised, the current rate at any given time isn’t that important because it won’t be the one that’s applied.
So as Tink says, the rate that’s shown in the ‘welcome to [country]’ notification when you enter the country should generally be sufficient, as a rough point of reference.
I can totally see why you’re asking but I wonder whether including the information would give users something new to worry about? Presumably most don’t think about the FX rate’s movements having travelled somewhere & I’m not sure they need to.
For the power users like yourself, there’s always Google or the app’s that show the FX rates..