How to draw vertical lines across chart?

[amaunz]

What is the recommended way for drawing vertical lines at given x positions?
I have a chart with an x axis like this:

type: 'Numeric', 
position: 'bottom'

where the values range from 0 to 1. I want to draw thin vertical lines across the whole chart area from top to bottom at 0.25, 0.50, and 0.75. How do I do that? Must I use sprites, and if so, how can I determine their coordinates?

Thanks

[scottmartin]

Please see the following:
https://fiddle.sencha.com/#fiddle/15d

[amaunz]

Great, thanks.