User design for many items over a time frame

I would like to represent a sequence of user events (for ex. login events) under a graph. This would typically hold zero to infinite entries.

To represent users I intend to use small icons of, say, roughly 64x64. But how do I represent them with good UX over a timeline ? If for example I simply draw them over a line representing time, there may be blanks and overlaps. If i represent them in fixed space sequence, I loose the information of how much time elapses between events and may need to re-include that anyhow. I'm thinking I could make use of more available screen space.

Should I keep any of these two designs ? Can they be improved ? Can a completely different fit ? I'm open to all and any suggestion.