As in this article, chart.js allows to use plugins

https://www.chartjs.org/docs/latest/samples/legend/html.html

But I can’t find a way to use it in chartkick gem with rails.

I tried calling the plugin with

Chartkick.eachChart(function (chart) {
        let options = chart.getOptions(), chart_data = options["library"]["data"];
        let plugins = {
            htmlLegend: {
                containerID: 'legend-container',
            },
            legend: {
                display: false
            }
        }
        if (options['darkColors'] === true) options["dataset"]["backgroundColor"] = options['colors'];
        options["library"]["plugins"] = plugins
        chart.setOptions(options);

        if (options.stacked) {
            chart["plugins"]=[htmlLegendPlugin];
            chart.chart.update();
        }
    });

where htmlLegendPlugin is the plugin which is given in the link.