ng generate will generate

filename <name>.<type>.ts content:

export interface <name> { //camel case

}

ie

ng generate interface Itest sometype generates file name itest.sometype.ts content

export interface Itest {
}

Using Angular CLI,

ng g i filename

will create a file by the name filename.ts and will have export interface filename {} in it

Like so :

interface GraphDatas {
  firstTemperature: Period
  internalTemperature: {
    min: number;
    max: number;
  };
  secondTemperature: Period;
  thresholdAlerts: Threshold;
}

interface Period {
  currentPeriod: number;
  previousPeriod: any;  // TO DO
}

interface Threshold extends Period {
  hasBeenRead: number;
}

You will need to export the interface you want to use in your typescript file after that :

export interface test{
  listComputedDatas: GraphDatas;
}

And in your code :

import {test} from './pathToInterface';

...

randomProperty: test;
// Typescript should detect properties such as 
// randomProperty.listComputedDatas.internalTemperature.min

EDIT : You will be faster creating your own file manually than doing the angular-cli command...