How to draw a Stroke under each line

I'm going more for a background look rather than text-underline. 

I've retrieved the text lines through the following code:

                textContainerManager = textDisplay.mx_internal::textContainerManager;

                if (textDisplay.getStyle("lineHeight")) {

                    lineHeightRatio = parseInt(textDisplay.getStyle("lineHeight"))/100;

                }

                textLines = new Vector.<DisplayObject>(linesLength);

                linesLength = textContainerManager.numLines;

                linesLength = textContainerManager.tlf_internal::getActualNumLines();

The linesLength using numLines usually returns a smaller number than getActualNumLines(). I'm guessing because the non-visible lines are being reused.

Right now I'm using the graphics object of parent Spark Group container and draw lines on that by iterating through the text lines and getting the y value as you stated. The problem I'm having with this is to know when to draw the lines and if the TextLine is null then extrapolating out the correct coordinates, which I've done to some success.

Another cleaner approach I was thinking about is to extend TextLine where in the updateDisplayList method draw the line and then apply the ExtendedTextLine to the TextLineFactory(?)? I looked into it and could not find it. My skill is in Flex component lifecycle so I don't know if TextLine has a updateDisplayList method. Is this the right approach to solve this problem?