Graph bandwidth doesn't correspond with values

[fleon]

I'm having an issue where graph values don't correspond with numerical values. For example, i am monitoring bandwidth for eth ports on my firewall, and supposedly the maximum value in a period is 6 Mbps, but the graph only displays values up to 3 Mbps.

I opened a bug (https://support.zabbix.com/browse/ZBX-11074) and i was told that i didn't create the graph properly since i used AVG values.

From the bug linked above, here's a screenshot of my graph with AVG values:

https://support.zabbix.com/secure/attachment/44013/zabbix.png

And here's one after i modified the graph to use ALL values:

https://support.zabbix.com/secure/attachment/44028/zabbix.png

As you can see, they look exactly the same.
The values InOctets / OutOctets are being read once per minute. If there is just one value per read, avg and all shouldn't differ, hence the graphs.

If you look closely at the graphs you can see i set a zoom filter of 16 days (because we started having an issue on july 26). If i put the zoom level back to the default (dynamic 1 hour) then the numerical values do correspond with the graph pixels.

Am i wrong? Is there a bug? Please advice. This is on zabbix 3.04 on debian 8 fully updated, installed from the official zabbix repository.

[Atsushi]

If you want to plot the maximum value,
please try to select the max in the function of the items for the graph.

[fleon]

You are right, setting max indeed shows the highest value, however this has led to some interesting events, which i have catalogued in the bug tracker at https://support.zabbix.com/browse/ZBX-11074

As i say there, i did a test. I set up the zoom level to only show 1 hour of data, so i could see each minute plotted on the graph. Since i collect only 1 value per minute, this means that avg, min, max and all should show the same graph value per minute, since there is just 1 value per period!

However, what i didn't realize is that the graph values were being down sampled because it was trend data (older than 7 days). This means that the actual per minute values are missing therefore zabbix can't show the actual values properly.

Lesson learned