How to pass ax.vlines() or plt.axvline() to charting figure?

[BlackArbsCEO]

Hey great work overall. I'm trying to figure out how to customize the plots even more by accessing the axis for the charting figure and adding vertical lines. As an example of what doesn't work:

ax.axvline(x=threshold_buy_dates[0], ymin=ax2ymin, ymax=ax2ymax, color='limegreen', linewidth= 2, alpha=.75)

threshold_buy_dates is a pandas index object with dates when certain timeseries events have occurred.
However plotting horizontal lines does work:

ax = fig.get_ax(2)
ax.axhline(1, color='orange', linestyle='-.', linewidth= 2)

Any suggestions on what to do would be helpful. I don't mind tinkering with the code in a fork but I would benefit greatly if you could point me in the right direction as to where to look to make changes if changes are necessary. Thanks!