Display vertical line on stacked line chart already using primary and secondary axises

I've got a step chart. It's built from a stacked line chart with double entries at the change points to spoof right angles. (I have attached a dummy copy).

The x-axis values are dates. It extends into the past and future. So, from mid August to end of September or so, when right now is the beginning of September.

The y-values are counts of widgets. So y1 is the count of red widgets, and y2 is the count of blue widgets. It's a stacked line chart so they add on top of each other. That's good, I want that. But, y0 is the goal line for the target number of widgets; I want it to display separately from all the rest of the widgets, without stacking with them. So I put y0 on the secondary axis and just manually forced both axises to be the same.

But now I want to go in and add a vertical line at today's date. When I'm already using the secondary axis. I want like a tertiary axis or something.

Anyway, what's your advice?