Question asked on groups.google.com/g/ets-users/about following Jonathan's advice.
Answer from Corran Webster :
you probably want a CoordinateLineOverlay. Unfortunately, it seems to be missing from the overlay documentation, but the class definition is here: https://github.com/enthought/chaco/blob/master/chaco/overlays/coordinate_line_overlay.py#L14 and an example of how to use it is here: https://github.com/enthought/chaco/blob/b89b2e5c0935d79eef9e2928d32356c4043dc624/examples/demo/coordinate_line_overlay_demo.py
There is also the LineInspector tool if you want a line which tracks the mouse position, and of course the Grid class for drawing multiple vertical/horizontal gridlines.
-- Corran