Well. To more questions:
1) Do you want to see these lines for the chosen session only (e.g. NY or London only) or for all currently active sessions?
at the minimum, i need the last session lines to be visible in oder to reference the current session's evolving price action.
lets establish the trading day as the point of change. when a new trading day begins, all lines from each of the previously day's active sessions are erased. at the trading day progresses through the various active sessions, the lines are drawn for each active session and shall remain visible until the end of the the current trading day.
2) What to do with the lines when the session ends? Should line disappears until the next session begins?
the lines should be left visible for each active session until the end of the trading day.