Plain text widgets used at dashboards does not support zoom functionality. It is not possible to view historical values  at plain text widgets. Only latest data is displayed at those plain text widgets.

Steps to reproduce:

1. Create New dashboard "Test"
2. Add single widget "Plain text" type
3. Save Dashboard "Test"
4. At dashboard "Test" it is not possible to use Zoom filter
5. Add another widget (Graph) to dashboard "Test", and save dashboard
6. At dashboard "Test"it  is  possible to use Zoom filter but it does not affect "Plain text" widget. Only latest data at "Plain text" widget is displayed. Note that Graph widget is filterable with Zoom filter as expected.

Expected:
"Plain text" widgets also should be filterable with zoom filter, just like Graph widgets. It should be possible  to see historical data at "Plain text" widgets using Zoom filtering at dashboards. My point of view is that dashboards are not used just to display latest data, but also it is interesting to see dashboards as they were at some point in time. 

At old zabbix infrastructure (Zabbix 2.0.5) there was no dashboards, but there were 'screens' functionality.

'screens' functionality had no problems with zooming of "text items".  As I understood, the dashboards functionality (at new zabbix versions) is successor of screens functionality. So I suppose that this reported issue should be considered for fixing. 

This issue is related with same issue that exists at zabbix server version 5.4, reported as  ZBX-19842