Apply custom CSS to the text drawn within a dygraph. See the CSS documentation on the dygraphs website for additional details on which styles are available.
Note
See the CSS Styling article on the dygraphs for R website for additional details.
Note that CSS styles are global so will affect all dygraphs on a given web page. This also implies that for a page with multiple plots you only need to specify styles for the first one (alternatively you can just add them directly to the page by other means).