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Yes, a watermark image can be added to dygraphs. One way to do this is by using the canvas.d context to draw the watermark image on top of the dygraph. Here's an example of how to add a watermark image to a dygraph:

// Create a dygraph
var data = [
  [new Date(2021, 1, 1), 1],
  [new Date(2021, 1, 2), 2],
  [new Date(2021, 1, 3), 3],
  [new Date(2021, 1, 4), 4],
  [new Date(2021, 1, 5), 5],
];
var options = {
  width: 600,
  height: 300,
};
var graph = new Dygraph(document.getElementById("graph"), data, options);

// Add a watermark image
var watermarkImage = new Image();
watermarkImage.src = "watermark.png";
watermarkImage.onload = function() {
  var canvas = graph.canvas_;
  var ctx = canvas.d.getContext("2d");
  ctx.globalAlpha = 0.5; // Set the opacity of the watermark
  ctx.drawImage(watermarkImage, 0, 0, canvas.width, canvas.height);
};

In this example, we create a dygraph and then load a watermark image. We use the onload event of the image to draw it on top of the dygraph using the canvas.d context. We set the opacity of the watermark to 0.5 to make it semi-transparent.