Very promising @danielfomin96, thanks for the work!
At present, there is some cross-play at work with another plugin, Kity Minder (v1.0.15 in use).
When your plugin (Markmap: 1.0.0) is installed, Kity Minder changes its behaviour and instead of opening its normal interface to create a diagram, it appears to output a diagram using your plugin.
Joplin 2.10.13 (prod, linux)