Plot a date as a horizontal line across graph,

Hello! 

I am wanting to plot a single line that would extend across a graph(the line would repree, for contact the graph is an epi curve representing the number of positive COVID cases along with a case rate line. So I want to add is the date in which the facility notified me of the first positive. from what I could father I can do this by using 4 cells in a workbook, have 1 of the cells reference the date, one cell with 0. but haven't gotten passed that. here is the graph below for context and visualization.