[FIX] OWProjectionWidgetBase: Update when domain is changed

[janezd]

Fixes #3944.

@VesnaT, please confirm this is the right way to do it and that nothing else is needed - also considering possible other similar bugs.

[codecov]

Codecov Report

Merging #3946 into master will decrease coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3946      +/-   ##
==========================================
- Coverage   85.21%   85.13%   -0.09%     
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  Files         382      378       -4     
  Lines       67590    67117     -473     
==========================================
- Hits        57600    57137     -463     
+ Misses       9990     9980      -10

[VesnaT]

No, this is not the right way to solve the issue. Setting _invalidated to True, causes recomputation of the embedding (in this - Scatterplot's - case there is no calculation and the replotting is not as wasteful as in other widgets that inherit from OWDataProjectionWidget, but should still be avoided).

1. One of the solutions to the issue, could be to call OWScatterPlotGraph.update_axes in OWScatterPlot.handleNewSignals (not the best one because the axes are always updated):

super().handleNewSignals()
self.graph.update_axes()
self._vizrank_color_change()

2. Another one (probably the best one) is to introduce a new flag self._domain_invalidated in OWDataProjectionWidget and call the OWScatterPlotGraph.update_axes only if it is set to True.

self._domain_invalidated = not (
                data_existed and self.data is not None and
                effective_data.domain.checksum()
                == self.effective_data.domain.checksum())

3. And if you want a general solution (even though I'd rather not plot data again if not changed):

def handleNewSignals(self):
       self._handle_subset_data()
       if self._invalidated:
           self._invalidated = False
           self.setup_plot()
       else:
           self.graph.update_coordinates()
           self.graph.update_point_props()
       self.commit()

[janezd]

My reasoning was that renaming variables is so uncommon that we can afford to update the entire graph. Still, your solutions are better. I would prefer the first one; updating axes costs nothing and simplicity counts. But I implemented the second because other widgets may also need to know that domain has changed.