Adding gaugings to an existing no-gauging posterior RC component results in weird behaviors

[IvanHeriver]

The identified issues are in the main plot:

• gaugings are added to the back (instead of the front) and are thus hidden by the uncertainty envelops
• Gauging legend is not set correctly.

Fixing this issue likely requires a full rewrite of the component so the existing plot items are tracked instead of the raw data so adding a new plot item can be properly done (initialization, addition in the right location).