Is there a way to mark multiple points in chart from list with single values and tuples?

Question

I have a label list and I would like to use this list to mark points in graphs.

label_list = [
    pd.to_datetime("2021-07-21"),
    pd.to_datetime("2019-11-29"), 
    pd.to_datetime("2019-01-09"),
    (pd.to_datetime("2020-03-31"), pd.to_datetime("2019-06-27")),
    pd.to_datetime("2019-06-03"), 
    pd.to_datetime("2020-10-16"), 
    (pd.to_datetime("2021-01-04"), pd.to_datetime("2021-08-11")),
    (pd.to_datetime("2021-05-08"), pd.to_datetime("2019-02-13")),
    (pd.to_datetime("2020-06-11"), pd.to_datetime("2019-07-25"),pd.to_datetime("2018-11-27"))]

I have created 8 graphs using pyplot and for loop and saved them with

plt.savefig(r'...\Image_{0}.png'.format(i))

I have tried using list comprehension

[plt.axvline(_x, linewidth=1, color=colours) for _x in label_list]

but I keep getting "ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()" Is there a way to do it?