JSFiddle - React, Tailwind, and code Playground

HTML

<h1>Chart One</h1>
<canvas id="LineChart" height="100"></canvas>

<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/3.5.0/chart.min.js"
    integrity="sha512-asxKqQghC1oBShyhiBwA+YgotaSYKxGP1rcSYTDrB0U6DxwlJjU59B67U8+5/++uFjcuVM8Hh5cokLjZlhm3Vg=="
    crossorigin="anonymous" referrerpolicy="no-referrer"></script>
<script src="https://cdn.jsdelivr.net/npm/chartjs-plugin-datalabels@2"></script>

JavaScript

var ctx = document.getElementById('LineChart').getContext("2d");
let width, height, gradient;

function getGradient(ctx, chartArea) {
  const chartWidth = chartArea.right - chartArea.left;
  const chartHeight = chartArea.bottom - chartArea.top;

  if (gradient === null || width !== chartWidth || height !== chartHeight) {
    width = chartWidth;
    height = chartHeight;
    let y = 27; // I want this to be the pixel for value I pass, instead of hard value

    // but I can't select y-axis like this:
    // var yaxis=y.scales[ 'y-axis-0' ];
    // as it says undefined, and if I do like this : 
    //  const { ctx, canvas, scales: { x, y } }=chart;
    // var xaxis=x.scales[ 'x-axis-0' ];
    // It says chart not defined, which I think is because I am not making this a function and passing chart, and I can't figure out how to do it right.


    gradient = ctx.createLinearGradient(0, chartArea.bottom, 0, chartArea.top);
    gradient.addColorStop(y / chartHeight, 'black');
    gradient.addColorStop(0.6, 'yellow');

    gradient.addColorStop(0.9, 'red');

    gradient = ctx.createLinearGradient(0, chartArea.bottom, 0, chartArea.top);
    gradient.addColorStop(y / chartHeight, 'black');
    gradient.addColorStop(0.6, 'yellow');

    gradient.addColorStop(0.9, 'red');
  }
  return gradient;
}
var myChart = new Chart(ctx, {
  type: 'line',
  data: {
    labels: ['10', '20', '30', '40', '50'],
    datasets: [{
      label: 'Hello',
      data: ['1', '2', '3', '4', '5'],
      backgroundColor: 'rgba(255, 255, 78, 1)',
      borderColor: function(context, options) {
        const chart = context.chart;
        const {
          ctx,
          chartArea
        } = chart;
        if (!chartArea) {
          return null;
        }
        return getGradient(ctx, chartArea);
      },
    }]
  },
});