Highlight points on an Excel Line Graph according to a list

I have 2 data series, which records how much a user is meditating/attentive (out of 100) plotted onto a graph. The x axis is the number of seconds since the start of the experiment, and the y axis shows the value for meditation/attention at that point of time.

I have a 3rd set of data that is a series of key timestamps during the experiment (not exactly matching the timestamps from attention/meditation values).

I want to create a graph where you can compare how the attention/meditation values change at the key points

Whether the key points are highlighted by a line or dots I don't care. I tried adding the 3rd data set as a secondary axis, but it still uses the original x-axis of the main graph and I don't know how to make excel do what I want.

Thanks in advance