As soon as you import your LineChart.js module you are importing the Elm runtime as a global. This should work for you:
import React, { Component } from "react";
import REC from 'react-elm-components'
import LineChart from "./elm/LineChart";
export default class Graph extends React.Component {
render() {
return (
<div className="time-container">
<div>
<h1>Chart: </h1>
<REC src={Elm.Main} />
</div>
</div>
);
}
}
When you have multiple elm applications you should compile them into one file: elm make LineChart.elm BarChart.elm PieChart.elm --output=MainElm.js.
Also name your elm applications according to the filenames: module LineChart exposing ... so you can refer to them as Elm.LineChart and Elm.BarChart in your react components <REC src={Elm.LineChart} />.