I ma trying to insert inside the graph a square where I would like to type "something"

Hello, your code is particularly useful as forum users will fail to be able to run it by virtue of not having your private objects. Data_m2 etc. See the reprex guide to overcome this.

In general though , the solution would be to annotate with a geom_text or geom_label depending on the required effect. see for example:
How to annotate a plot in ggplot2 – the R Graph Gallery (r-graph-gallery.com)