Calculando a soma diária cumulativa no PostgreSQL

Você pode usar as funções de agregação e janela juntas:

select date_trunc('day', date_created) as day_created,
       location_state,
       count(*) opp_count,
       sum(count(*)) over (partition by location_state order by min(date_created)) as cumulative_opps_received
from t
group by day_created
order by location_state, day_created;

Você pode dividir se quiser a porcentagem:

select date_trunc('day', date_created) as day_created,
       location_state,
       count(*) opp_count,
       sum(count(*)) over (partition by location_state order by min(date_created))  as cumulative_opps_received,
       (sum(count(*)) over (partition by location_state order by min(date_created)) * 1.0 /
        sum(count(*)) over (partition by location_state)
       ) as cumulative_ratio
from t
group by day_created
order by location_state, day_created;