I want to display a list of 7 HDV trends on a single page. Is it possible to extend the page canvas in order to fit them and scroll down ?

There's nothing stopping you from changing the page size in the Idea Studio and drawing the HDV widgets, so I assume that you're looking for a way to do this dynamically, adding up to 7 widgets as needed.
(Although, I have to wonder why this is better than separating the graphs in one HDV...)
The solution is to do it the same way a station tag manages multiple pumps. Create them all on the page and then use expressions in the Opacity property to hide the ones you don't need in a given instance.

Hi Andrew,
It s true, the page size can be changed in the Idea Studio but the canvas will show a dotted line ... what passes that line will not show in the operator view (Runtime).
Somebody suggested to go to Application Settings and change Application Window behavior to --> Fixed size window and Application Window Size - Width / Height as desired.

It s working ... but when i scroll Up / Down, the image elements get scrambled for 1 second or more, like in this video
https://drive.google.com/file/d/1x5jpOuAStQ9NQXWbaoT_1M0CC4yJE_UG/view?usp=sharing