You can use https://www.image-charts.com/ with this you can send your config as as an api request and you get the image back so you will have to call this on your server to save it.
Example: https://image-charts.com/chart.js/2.8.0?bkg=white&c=%7B%22type%22:%22line%22,%22data%22:%7B%22datasets%22:%5B%7B%22type%22:%22line%22,%22fill%22:false,%22backgroundColor%22:%22rgba(255,%2024,%201,%201)%22,%22borderColor%22:%22rgba(255,%2024,%201,%201)%22,%22pointRadius%22:0,%22data%22:%5B67.0,65.0,70.0,81.0,83.0,81.0,79.0,74.0,78.0,77.0%5D,%22label%22:%22Output%22%7D%5D,%22labels%22:%5B%2200%22,%2205%22,%2210%22,%2215%22,%2220%22,%2225%22,%2230%22%5D%7D,%22options%22:%7B%22tooltips%22:%7B%22intersect%22:false%7D,%22scales%22:%7B%22xAxes%22:%5B%7B%22gridLines%22:%7B%22display%22:false%7D%7D%5D,%22yAxes%22:%5B%7B%22gridLines%22:%7B%22display%22:false%7D%7D%5D%7D%7D%7D
