fixing Mermaid diagram's height

[daiyam]

This change allows to specify the desired height of the diagram (in percentage in vh).
It should be able to fix #2335.

For example:

```mermaid:50
graph TB
	c1-->c2
	c2-->c3
	c3-->c4
	c4-->c5
	c5-->c6
```

[daiyam]

Here is a screenshot of the change:

[daiyam]

A better fix is available at #2394