x-axis constant time

I am creating a line graph. I am plotting based upon date/time. I'd like to use a constant date across the x-axis. IE 8:00, 8:15, 8:30. Within the constant value there could be many points or no points. For example there could be a point at 8:01, 8:02, 8:05 and then a point at 8:35. Along the x-axis the label should remain constant at 8:00, 8:15...

Any ideas?

Thank you,

Michelle