Dygraphs already provides a two options, drawPoints and pointSize, which will draw a small circle at every data point. Dygraphs will also draw points when hovering near a data set, and that circles size is determined by the option highlightCircleSize.
But who wants to be stuck with circles?
The drawPointCallback and drawHighlightPointCallback options both accept a function which draws the shape you want right on the canvas. As a starting point, Dygraphs now supplies a set of basic images. You can see all of them below:
Dygraph.Circles.DEFAULT Dygraph.Circles.TRIANGLE Dygraph.Circles.SQUARE Dygraph.Circles.DIAMOND Dygraph.Circles.PENTAGON Dygraph.Circles.HEXAGON Dygraph.Circles.CIRCLE Dygraph.Circles.STAR Dygraph.Circles.PLUS Dygraph.Circles.EX
The demo, which can be found here, also demonstrates drawing your own custom points, by drawing adorable little smiling and frowning faces.