There are two ways you can add a watermark:
backgroundImageUrlChart.js plugin. This will place an image in the background of your chart. You can supply a watermark image that contains, for example, the logo of your company. See documentation and an example chart.
Use the watermark API. Supply the chart URL as the
mainImageUrl, and supply a separate watermark image as
markImageUrl. This option is more customizable.
Here’s an example chart created using the watermark API technique: