I know this might be a very mathematically retarded question, but I am trying to find out the price of ETH and the altcoin at the time I bought them from completed order history. All it shows is the ratio of ETH/Altcoin, number of units got and bid/ask etc.
It depends what you mean by "price".
All it shows is the ratio of ETH/Altcoin...
That is the price. The cost of one currency in another.
If by price you mean "how much it was worth in dollars1", then in my opinion it's time to start re-thinking - or at least expanding - your definition of price. As with the "value" of one cryptocurrency in relation to another, the "value" of a dollar is always changing, just perhaps less so. When you see the value of your cryptocurrency rising, how do you know it isn't because the value of the dollar is decreasing? And so on.
Your next problem is that different exchanges will trade at different "prices". Poloniex prices won't be the same as Kraken prices - they're two distinct markets, and the price is set by market demand.
In a less abstract and pedantic world you could find the value of each of the currencies in the pair, in dollars, at that particular time.
To do this you could look at the ETH-USD and Altcoin-USD charts on the exchange you used, and work out the per-unit cost - e.g. one ETH was worth X dollars, one altcoin was worth Y dollars.
[1I've used dollars in this example, but insert your own fiat currency accordingly.]