How can I group counts by quarter hour in a chart?

I want to create a timeline of records to understand what was the busiest portion of a day. I created a formula to break record timestamps down to quarter-hours to reduce noise (but am open to a more streamlined way).

Looking at other answers here, I see that a Pivot Table can take the rows of quarter-hours and total amounts and give me one row per quarter-hour with a count of records, and that an XY Scatter is the chart type to use. However, instead of getting count (Y) by time/quarter-hour (X), I get record/row number along the X-axis. What am I doing wrong? (I’m guessing it’s something about using PTs with charts, but I have little experience with the former.)

example: timeline.ods