I was able to find out my flaw and posting here in case anyone else faces the same issue. If using time data, your X value for which you want to plot a vertical line must be of the same type as the X-axis. In my case, the X value for inc_time was of datetime type, while the X axis is of datetime.time. If your data entries use date and time, you can extract the time using the .strptime() method.
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