[FIX] Scatter Plot: Replot when input features change

[VesnaT]

Issue

Scatter Plot widget does not always refresh the plot on input features change. To obtain this behaviour the features must be connected before the data (see the included test).

To reproduce, open the attached workflow and change the selection in the Correlations widget. The plot in Scatter Plot does not change accordingly.
untitled.ows.zip

Description of changes

• introduce _attrs_invalidated variable in Scatter Plot widget

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #5837 (589345f) into master (5a57006) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5837      +/-   ##
==========================================
+ Coverage   86.20%   86.24%   +0.03%     
==========================================
  Files         316      315       -1     
  Lines       66627    66675      +48     
==========================================
+ Hits        57437    57504      +67     
+ Misses       9190     9171      -19     

[janezd]

Just a though: what about renaming _attrs_invalidated to _xy_invalidated or even _xy_changed?

Currently, there exist _invalidated and _domain_invalidated, the former referring to the change of effective data and the latter to change of variables that appear in effective data. Both names are not the most informative (I suppose that because we did not expect to have multiple "invalidates".) We can't rename the existing attributes, but the newcomer could have a more informative name.

If you prefer to keep _attrs_invalidate, say so, and I merge.

[VesnaT]

I liked the suggestion so I changed the name.