SPA webapp for plotting data with angular and python
I don't see a problem with your approach except that because you have real-time data, I would encourage you to go with some kind of WebSockets approach, like Socket.io on Node and the front end. The reason why I say this is because the alternative approach, which is long-polling, involved a lot of HTTP traffic back and forth between your server and client, which is a performance bottleneck.
Angular is perfectly fine for this, as you will not need to manually update your model data on the front end, thanks to two-way data binding.
There are many charting frameworks and libraries like D3.js and HighCharts that can be plugged into your front end to chart your data, use it according to your liking.
I want to write an app for plotting various data (cpu, ram,disk etc.) from Linux machines.
On the client side:
Data will be collected via a python script and saved to a database (on a remote server) eg.: In each second create an entry in a mongodb collection with: session identifier,cpu used, ram,iops and their values. This data will be written in sessions of a few hours (so ~25K-50K entries per session)
On the server side:
Data will be processed having the 'session' identified, plotted and saved to a cpu graph png/ram graph png etc. Also it will write to a separate collection in mongodb identification that will be used to gather and present this data in a webpage. The page will have the possibility to start the client on the remote machine.
Is this approach optimal? Is there a better but simple way to store the data ? Can I make the page construct and display the session dynamically to be used for example to zoom. Will mongo be able to store/save hundreds of millions of entries like this ? 
I was thinking on using  angular + nodejs or angular + flask on the server and mongodb. I don't know flask or node, which will be easier to use for creating a simple REST.
My skill levels: python advanced, javascript/html/css medium, angularjs 1 beginner.