[metachronik, charts.e.cash] Correctly calculate cumulative agora usd volume

Summary:
The current approach is not actually cumulative, as it shows decreasing cumulative volume with decreasing price.

Calculate the USD value every day so we can properly calculate the cumulative value.

Test Plan:
npm test, CI, check screenshots.

current prod (does not make sense, cumulative volume cannot decrease):

after this diff:

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18882