Q:

Chart.js Radar background color of background

anyone did know if there is possible to change background area of a Radar like the image, i poster?

A:

I had the same issue, looks like the documentations isn't friendly for these cases, I based my solution on these examples:

https://github.com/chartjs/Chart.js/issues/3956#issuecomment-287080840
https://www.html5canvastutorials.com/tutorials/html5-canvas-circles/

My options object looks like this:
options: {
    ...
    chartArea: { backgroundColor: 'red' },
    ...
}

I used the hook function beforeDraw in order to draw the background-color behind the chart and it looks like this:
Chart.pluginService.register({
    beforeDraw: chart => {
        const { ctx, scale, config } = chart
        const { xCenter, yCenter, drawingArea: radius } = scale

        ctx.save()
        ctx.arc(xCenter, yCenter, radius, 0, Math.PI * 2)
        ctx.fillStyle = config.options.chartArea.backgroundColor
        ctx.fill()
        ctx.restore()
    }
});

In beforeDraw function you can catch options object with your custom attributes.
You can check full list of hooks here

