Add Dynamically Vertical Line To Chart Without User Intervention After Initial Setup Of Workbook?

I am trying to add a vertical line dynamically to a chart. What I mean is I have a chart that will have a line type chart on it and I want to add a vertical line to it based on the output of one cell in that same workbook. I am making this workbook for use by end-users so I can't ask the end-users to make a bunch of modifications to the sheet or or chart. What I do has to just work when they use the workbook.

The line chart has an X axis of time and a Y axis of items sorted. (This is the part of the chart I have done already.) I want to add a vertical line fed from a cell which will have a time fed from a cell on the workbook. For example the line chart goes from 9:00 to 16:00 and the vertical line might be at 13:00. The cell feeding the vertical line is subject to change based on the other inputs on the workbook so that's why I say the vertical line has to be dynamic as the vertical line could be anywhere between 9:00 to 16:00 or it might even be that I can't place the vertical line because the feeding cell is outside the range of 9:00 to 16:00.

I have seen a number of ways to do this but none that are dynamic and automatic and don't require end user to adjust the workbook to make the vertical line at the correct on the time scale.