[ENH] OWBoxPlot: Show missing values

[PrimozGodec]

Issue

Fixes #4128

Description of changes

• In this PR we extend the box plot widget to show missing values for discrete variables.
• We update the contingency to have three additional parameters to hold number of unknowns for each column, row and combined unknowns. Also, calls of contingency around Orange were updated.

Includes

• Code changes
• Tests
• Documentation

[janezd]

We discussed this. Yes it would make sense to group by having/not having missing values. Good idea.

[codecov]

Codecov Report

Merging #4135 into master will increase coverage by 0.02%.
The diff coverage is 95.23%.

@@            Coverage Diff             @@
##           master    #4135      +/-   ##
==========================================
+ Coverage   85.71%   85.73%   +0.02%     
==========================================
  Files         389      389              
  Lines       69831    69832       +1     
==========================================
+ Hits        59856    59872      +16     
+ Misses       9975     9960      -15

[PrimozGodec]

It is ready now.

[ajdapretnar]

Hm. This doesn't seem to work on heart_disease... 🤔

Traceback (most recent call last):
  File "/Users/ajda/miniconda3/envs/o3/lib/python3.7/site-packages/orangecanvas/scheme/signalmanager.py", line 718, in __process_next
    self.process_queued()
  File "/Users/ajda/miniconda3/envs/o3/lib/python3.7/site-packages/orangecanvas/scheme/signalmanager.py", line 476, in process_queued
    self.process_node(node_update_front[0])
  File "/Users/ajda/miniconda3/envs/o3/lib/python3.7/site-packages/orangecanvas/scheme/signalmanager.py", line 523, in process_node
    self.send_to_node(node, signals_in)
  File "/Users/ajda/miniconda3/envs/o3/lib/python3.7/site-packages/orangewidget/workflow/widgetsscheme.py", line 775, in send_to_node
    self.process_signals_for_widget(node, widget, signals)
  File "/Users/ajda/miniconda3/envs/o3/lib/python3.7/site-packages/orangewidget/workflow/widgetsscheme.py", line 809, in process_signals_for_widget
    handler(*args)
  File "/Users/ajda/orange/orange3/Orange/widgets/visualize/owboxplot.py", line 318, in set_data
    self.grouping_changed()
  File "/Users/ajda/orange/orange3/Orange/widgets/visualize/owboxplot.py", line 392, in grouping_changed
    self.attr_changed()
  File "/Users/ajda/orange/orange3/Orange/widgets/visualize/owboxplot.py", line 402, in attr_changed
    self.compute_box_data()
  File "/Users/ajda/orange/orange3/Orange/widgets/visualize/owboxplot.py", line 427, in compute_box_data
    dataset, attr, self.group_var)
  File "/Users/ajda/orange/orange3/Orange/statistics/contingency.py", line 286, in get_contingency
    dat, col_variable, row_variable, col_unknowns, row_unknowns)
  File "/Users/ajda/orange/orange3/Orange/statistics/contingency.py", line 194, in __init__
    self.from_data(dat, col_variable, row_variable)
  File "/Users/ajda/orange/orange3/Orange/statistics/contingency.py", line 220, in from_data
    conts = data._compute_contingency([col_variable], row_variable)
  File "/Users/ajda/orange/orange3/Orange/data/table.py", line 1490, in _compute_contingency
    col_data, classes_, n_rows, W_)
  File "Orange/data/_contingency.pyx", line 11, in Orange.data._contingency.contingency_floatarray
    @cython.wraparound(False)
ValueError: Buffer dtype mismatch, expected 'intp_t' but got 'double'

[PrimozGodec]

It works for me. There is a change in the Cython code. It was compiled in .c code already but it needs to be compiled again for each computer (at MacOS the result of the compilation is .so file). The easiest way to do that is by running:

pip install -e . 

in the Orange folder when this PR is checked out.

[ajdapretnar]

Ah, important detail. 👌 Love the change!