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That demo is accessible in ZIP packages which you can download from ChartJS site
You can open corresponding file
DevExpressChartJS-13.2.7.zip\Demos\VizGallery\demos\Circular Gauges\Basic Elements\Palette for Ranges.html
And modify it.
You also can mofify code directly on page which you linked using online code editor (but you wonβt be able to save results)