It is not normal for contnous increase in Memory utlisation. However, it is fine over a short period of time, where resources are added to system (ex: users, learnt MAC-addresses, DHCP-leases..etc...) but after period of time they move away/ gets deleted, hence processes holding the memory will be released.
From the graph it looks like consistent increase over last 10 months. And if same trend continues it will eventually lead to memory exhaustion, resulting in issues such as crash etc....
As a precaution, opening a TAC case would be helpful, so that it can be looked upon proactively before landing into other issues...
Thanks,
-Vinay