[Chartjs]-React line chart background fill

You can set the drawTime of the fillerPlugin to beforeDraw so it draws the fill before chart.js draws any of its own components.

https://www.chartjs.org/docs/4.3.0/charts/area.html#configuration

new Chart(ctx, {
    data: {
        datasets: []
    },
    options: {
        plugins: {
            filler: {
                drawTime: 'beforeDraw'
            }
        }
    }
});