Avoid "shrinking" of the plot area if the legend is above/below the plot

[Yuri05]

This issue was fixed e.g. for PK Ratio plots (#758) but it still happens in MEanModel/Population workflows for many plot types (time profiles, residual plots, histograms, ...)
Plot area must keep its size for ALL plot types.

[pchelle]

Should be fixed by PR #924

[Yuri05]

For some images it works good; for many images it does not.
S. the report here: https://github.com/Yuri05/TestReport/tree/main/Report_07_Population_Pediatric_Raltegravir (Report inputs are in the same folder)

Some of the problematic images