Historical Price Data for STEEM, SBD and SteemPower

I have written a script that collates transaction history into a csv file as this spreadsheet shows.

What I would like to do is provide the USD equivalent of the STEEM, SDB, SteemPower or Vests that the API provides via some sort of calculation.

I envision this as a 2-step process. Convert SBD, STEEM or SteemPower to BTC and then convert BTC to USD, both based on the historical value on the day of the transaction.

The BTC -> USD component is "solved" - I will use the close from https://docs.gdax.com/#get-historic-rates

But the missing component is still Historical Price Data for STEEM, SBD and SteemPower and any help is appreciated.