Exchange rate dynamics tracking/plotting

The feature should allow user to track some currency pair, recording time-based log of exchange rates. Later, the user should be able to see the plot. This would allow user to monitor custom exchange pairs, e.g. `ATOM/DOT` which is not very common pair on exchanges.

No files, tests, or entry points are named. Start by identifying where currency pairs and exchange-rate history would be handled and where a plot could be displayed; done means users can track a custom pair, retain time-based rates, and view its plotted history.

Written by the indexing model from the issue text.