It seems that some UI was broken in version 6.0 (affected also 6.0.1 and 6.0.2).
It's impossible to save modified values of some attributes for the classic graphs.

Steps to reproduce:

1. Navigate to "Configuration" -> "Hosts" from the main menu of Zabbix web interface;
2. Click on the "Graphs" link for any host containing a classic graphs (non-inherited from the template);
3. Click on any graph with "Graph type"="Normal" (or just create a new graph, save and then open it for editing);
4. Change attributes "Draw style" and "Y axis side", possible - some other fields, click "Update" button, then open this graph for editing again.

Result:
You see the old values in fields "Draw style" and "Y axis side".
All other fields contain their new (modified) values.

Expected:
All attributes (including the "Draw style" and "Y axis side") should be saved.

Zabbix v5.0.x did not have this bug.