pydata inspired fixes

[OriolAbril]

First batch of fixes and improvements related to my pydata tutorial.
This is mostly a temporal workaround I think but I wanted to generate
ppc related plots for specific subsets of data. I did some changes in plot_ppc_dist
which was mostly there when given the right facetting strategy,
but most other plots will take non negligible work.

On the other hand, by using this helper function we can convert a multidimensional
variable into multiple independent variables which already plays well with most plots.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.64%. Comparing base (f4aa51e) to head (2303b00).

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HEAD has 1 upload less than BASE

Flag	BASE (f4aa51e)	HEAD (2303b00)
	4	3

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #204       +/-   ##
===========================================
- Coverage   73.38%   51.64%   -21.75%     
===========================================
  Files          20       20               
  Lines        2029     2037        +8     
===========================================
- Hits         1489     1052      -437     
- Misses        540      985      +445     

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