Chartjs-Why is the data from my API not displaying on my chart.js chart in React?

Your dataObject in axios res is not correct, you forgot intensity.
your Chart3.js have to look like:

import axios from "axios";
import React, {useState, useEffect} from "react";
import { Line } from "react-chartjs-2";
const Chart3 = () => {
  const [chartData, setChartData] = useState({});
  const [Forecast, setForecast] = useState([]);
  const [Actual, setActual] = useState([]);

  const chart = () => {
    let Fore = [];
    let Act = [];


    axios
      .get('https://api.carbonintensity.org.uk/intensity/2020-08-25T15:30Z/2020-09-10T17:00Z')
      .then(res => {
        console.log(res);
        for (const dataObj of res.data.data) {
          Fore.push(parseInt(dataObj.intensity.forecast));
          Act.push(parseInt(dataObj.intensity.actual));
        }
        setChartData({
          labels: Fore,
          datasets: [
            {
              label: "Forestreet",
              data: Act,
              backgroundColor: ["rgba(75, 192, 192, 0.6)"],
              borderWidth: 4
            }
          ]
        });
      })
      .catch(err => {
        console.log(err);
      });
    console.log(Fore, Act);
  };

  useEffect(() => {
    chart();
  }, []);
  return (
    <div className="App">
      <h1>Dankmemes</h1>
      <div style={{height:"500px", width:"500px"}}>
        <Line
          data={chartData}
          options={{
            responsive: true,
            title: { text: "ForeStreet", display: true },
            scales: {
              yAxes: [
                {
                  ticks: {
                    autoSkip: true,
                    maxTicksLimit: 10,
                    beginAtZero: true
                  },
                  gridLines: {
                    display: false
                  }
                }
              ],
              xAxes: [
                {
                  gridLines: {
                    display: false
                  }
                }
              ]
            }
          }}
        />
      </div>
    </div>
  );
};

export default Chart3;

the only difference is dataObject :
dataObj.intensity.forecast instead of dataObj.forecast
sandBox