We can conveniently do this employing labs, which lets us specify a title, subtitle, and caption argument:

cash_flow_report %>% 
ggplot(aes(x = y, y = cash_flow, group = x, colour = x))+
geom_point()+
geom_line()+
labs(title = "cash flows over time by region", subtitle="quarterly data from 2014 to Q2 2017")

Much, much better. Let's move on to the axis and legend labels.