[FIX] SOM: Fix crash when color is constant

[janezd]

Issue

Fixes #5858.

If the variable determining the color was constant, the binning set two thresholds, which were both removed, thus creating an empty list of thresholds and labels. This created problems later in the code.

Description of changes

Set a threshold and label.

Includes

• Code changes
• Tests

[codecov]

Codecov Report

Merging #5860 (c00df78) into master (269d7fa) will increase coverage by 0.00%.
The diff coverage is 97.29%.

@@           Coverage Diff           @@
##           master    #5860   +/-   ##
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  Coverage   86.29%   86.29%           
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  Files         315      315           
  Lines       66793    66815   +22     
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+ Hits        57638    57659   +21     
- Misses       9155     9156    +1     

[VesnaT]

The fix works, except when the constant is NaN:

[janezd]

The fix works, except when the constant is NaN:

Mostly this. :) https://github.com/biolab/orange3/blob/master/Orange/widgets/unsupervised/owsom.py#L574

But also another place.

[VesnaT]

node -> no

[janezd]

yes

[VesnaT]

Similarly, the plot can't be colored by a Time Variable.

[janezd]

Similarly, the plot can't be colored by a Time Variable.

This problem was actually in time_binning and would happen in any widget using this for discretization.

Which, by the way, reminds me: all widget should use time_binning/decimal_binning for discretization. E.g. Mosaic and Sieve, as well as all widgets that show color legends.