Choosing the correct database type for single type of entry
NoSQL databases are for unstructured data or unpredictably structured data that is often queried using full-text searching.  

Capturing time-series statistics for graphing sounds like a pretty well-structured data set to me.  What do you have, three tables?  Users, Servers and Measurements?  It sounds to me like you have data which is perfectly suited to a relational database.

It may be that you need to think about whether your relational database is going to be fully normalized or partially denormalized for reporting performance.  That's something you can decide is necessary once you do some load testing with production volumes in order to see whether denormalization is helpful.