Actionscript 3 :: Draw Line Segment On Chart Using Screen Coordinates?

I'm new to Flex, I've tried to do this using a CartesianDataCanvas inside backgroundElements in mxml, then using ActionScript to draw lines on the data canvas using moveTo(x1,y1) and lineTo(x2,y2), which draws a line between (x1,y1) and (x2,y2).

The problem with the above method is the tick mark length is a function of the chart area (which depends on the browser window size).

What I need is a method that:

(1) starts at a data coordinate (e.g. moveTo(x1,y1) works fine for this)

(2) draws a line to a screen coordinate (e.g. lineTo(x_screen, y_screen) where x_screen and y_screen are screen coordinates).

Is there anything that can accomplish this in Flex/AS3?

Alternatively, could I use screen coordinates for both steps above? If so, how to convert between screen and data coordinates? For example, is the upper right corner of the screen always a fixed data coordinate that I could reference is creating such a conversion?

Alternatively, could I save a five-pixel line in Illustrator and simply paste that image in the chart somehow? If so, how to paste it exactly at a data coordinate?