The graphs show only IN traffic for an interface, although there is both IN and OUT traffic flowing through that interface. Why's that?

NetFlow traffic accounting is ingress by default, which means that NetFlow accounts for only IN traffic across an interface. Hence, to see both IN and OUT traffic graphs for an interface, you need to enable NetFlow on all the interfaces through which traffic flows. Some IOS version also supports accounting of egress traffic. For example this feature can be used if you using edge router where one interface is connected in to your LAN and the second one is interface with encrypted traffic (crypto map).