How do I put a marker on a plot?

I have a Linear Acceleration plot, and have calculated the ImpactStart and ImpactEnd points, defined in terms of Time on the x-axis. These are identified a Time = 0.7579 and Time = 0.7598, respectively on the graph below.

I would like to add markers to my graph at these two points to visually identify the ImpactStart and ImpactEnd.

But the markers are not plotting in the correct location.

A copy of my m-script for this graph is attached.

I have been struggling with this for a couple of days. Would greatly appreciate any help.

Thanks, John