@Test
public void testGenerateChart() throws Exception {
    try (final WebClient webClient = new WebClient(BrowserVersion.CHROME)) {      
        webClient.getOptions().setCssEnabled(true);
        webClient.getOptions().setJavaScriptEnabled(true);
        webClient.waitForBackgroundJavaScript(25);
        final HtmlPage page = webClient.getPage("https://www.chartjs.org/samples/latest/charts/bar/vertical.html");
        Window window = page.getEnclosingWindow().getScriptableObject();      
        int i = 0;
        do {
            System.out.println("start animation " + i);
            i = window.animateAnimationsFrames();          
            System.out.println("end animation");
        } while (i > 0);
        HtmlCanvas canvas  = page.querySelector("#canvas");
        HTMLCanvasElement picelem = (HTMLCanvasElement) canvas.getScriptableObject();
        String png = picelem.toDataURL("image/png");
        System.out.println(png);
        byte[] decodedBytes = Base64.getDecoder().decode(png.substring(22));
        String filename="D:/temp/chart.png";
        FileUtils.writeByteArrayToFile(new File(filename), decodedBytes);      
    }
}

Real result:

Expected: