Zabbix can schedule time intervals of item execution with Custom intervals options.
If you use "Update interval (in sec) - 0" and only get "Custom intervals", Graphs showing dots instead of lines.

Steps to reproduce

• Update interval (in sec) : 0
• Custom intervals :
  TYPE - Scheduling, INTERVAL - wd1-5h9-22m/1 (execute 1 minute)

This case, I confirmed that zoom 5m ~ 1h graphs showing dots instead of lines.
Precisely, This is affected by the width size of the graph.