The dyCrosshair plugin draws a crosshair line over the point closest to the mouse when the user hovers over the graph. It has a "direction" option which is provided in the R wrapper function and then forwarded to the plugin using the "options" argument to dyPlugin.
Dygraph to add plugin to
Crosshair direction. Valid options are: "both", "horizontal", "vertical"
Dygraph with Crosshair plugin enabled
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