plotting a single line graph

I want to make a graph plotting points every 4 minutes on a horizontal axis
while plotting breathe per minute during exercise on a vertical axis.

How have you entered your into your spreadsheet?

Like this?
Time BPM
12:00 15
12:01 14
12:02 16
12:03 12
12:04 12
12:05 15

Or this?
Time BPM
12:00 15
12:04 13
12:08 16
12:12 14
12:16 12
12:20 13

Plotting the graph will depend a lot on how your data is arranged.