Steps to reproduce:

1. Send some multiple values to item at the same time (only nanoseconds differ) using command for ex:

    ./bin/zabbix_sender -z localhost -s HOST -k trap3 -o 15 && 
   ./bin/zabbix_sender -z localhost -s HOST -k trap3 -o 5 && 
   ./bin/zabbix_sender -z localhost -s HOST -k trap3 -o 10
   

2. Create a graph widget with this item in data set (use black color and 0 Transparency to see the dots sharply.
3. Try Approximation functions min, max, avg and all.

Choose min, max, avg - dot is seen on the graph - EXPECTED

Choose approximation "all" - not any dot or line seen on the graph - NOT EXPECTED

Expected:
Line connecting from 5 to 15 seen on the graph, reflecting all values for the item received at that time point.