How do the native data fields do the '__:__' display?

With a native data field on the watch (Forerunner 235) such as pace, when the field is zero (or null or infinity, or above some threshold; however you want to think of it), the display shows four underscores and a colon, e.g. __:__
The underscores are very thin (1 pixel). If I try to replicate this with a CIQ data field, using dc.drawText() and giving it the string '__:__', the underscores are quite fat (3 pixels?). To get the same native look, I resorted to just passing ':' to dc.drawText() and then using four dc.drawLine() calls to draw the underscores manually.
How do the native data fields show such thin underscores, is it something similar (drawing lines), or is there another glyph in the font set that is available?