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Currently there isn't a graph of the ripple trade values.
I'd like to know if each trade is recorded in the Ripple's equivalent of the blockchain, so that if in future someone wants to build such graph, it will be able to include old data instead of just newer one.
The ledger chain stores all transactions. So yes, you can rebuild the transaction history. But the validators don't need to store or serve the whole chain to operate. A new validator only needs the last closed ledger to validate new transactions. This is different from bitcoin, where nodes need the last output of the relevant public key to validate a new transaction (and that last output won't necessarily be on the last block).
In theory the ledger chain could be forgotten, as validators don't need the history, but probably some "history storage" services ala blockexplorer.com will appear. Some validators can find it useful to store the whole history too.
Since it's not really necessary for the network to work, I'm not sure what happens if a server doesn't serve the ledger history, but I would say the RPC call useful to query the history is this one: https://ripple.com/wiki/RPC_API#ledger_entry
