reason of discrepancy btw diff vs delta_n

[asdwe2108]

1. The reason why delta n differs b/w jiahao and liuyi: (a,b,c) v.s (b,c,d,e,e) 16831203027 41802068265 43736454825 521870063421 522033862841
2. How to add text on the top of histogram in ggplot?