Add a dot (marker) without y-axis value on the existing graph

I have a graph of my stocks portflio over some period of time, which looks like this:

I have a y-value for every 5 minutes, so x data is timestamps and y data is just numbers.
I also have a list with times of operations (when i bought or sold something), which looks like this:
I want to add a dot or some kind of a marker for every operation on my graph, but i don’t know how to do this, I don’t have y-values for it. And timestamps x-values are different from operations x-values, so i can’t just take y-values from an existing graph.
This is how I imagine it ideally, but for starters i just want to understand how to add my points on the graph.