Can I use dt ?

I want to write a dashed vertical line on the plot, then
``` ruby
Numo.gnuplot do
  set arrow: "from 10.0 ,0.0 to 10.0,140.0 nohead dashtype (10, 5)", lc_rgb:"red"
  plot xx, yy, w: :l
end
```
But I only got a plain red line.  How do I set the dashtype appropriately from numo/gnuplot?