docs: add notes for the implementation

Author: ngfelixl

Diff for: NOTES.md

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+These are notes for the stream implementation of Nodeplotlib.
+
+## General
+- A plot window (**apps/web**) tries to connect to the server via a realtime api (e.g. websockets).
+- The server recognizes the count of connected apps.
+- If the user executes the `plot` function several times, it will only open a window if there is no
+  open connection to a **apps/web**.
+
+## Server lifecycle
+- The server starts with the execution of the `plot` function if there is no active server running.
+- The server stops if all **apps/web** are disconnected (and there were connections before).
+
+## The plot function
+- The plot function can either handle a `Plot` or an `Observable<Plot>`.
+  - It creates an `Observable<Plot>` by using Rxjs' `of` observable constructor.
+- The plot streams are saved in a Plots Set.
+- If there is an active **apps/web** that listens to the server, it subscribes to all Plots in the Set.
+- It does not submit a whole "plots" object, but rather submits all plots one by one. The reason is
+  realtime data, for which only the updated plot should be transmitted.
+- If all **apps/web** are disconnected, it should close the observable subscriptions of the plots and close
+  the server as mentioned in the **server lifecycle** section.
+
+## The stack function
+- Is the `stack` function really needed? Stack served the purpose that only one window opens which
+  could display several plots.
+
+## Backlog
+- The user can remove plots from the frontend. If that happened it submits a message to the
+  backend so that the subscription can be cancelled and the plot stream can be removed from the plots set.
+
+## Frontend only
+- The user has the possibility to rearrange plots per drag and drop.
+- The user can resize the individual plot windows.