Bug: Single line diagram canva is not full height

[michelguerin]

Describe the bug On the single line diagram plugin, the viewport is not full height with simple or little diagrams.

To Reproduce Steps to reproduce the behavior:

1. Open a new file with single line diagram that is small.
2. Go to Single line diagram
3. Move it around
4. It disappear

Expected behavior The single line diagram container should be full height, so that diagrams don't disappear.

Screenshots

[michelguerin]

This issue might be linked to #472