[ENH] Remove discrete attributes from scatter plot's axes

[janezd]

Issue

Fixes #3318.

Includes

• Code changes
• Modified tests

[codecov]

Codecov Report

Merging #3434 into master will decrease coverage by 0.03%.
The diff coverage is 96.87%.

@@            Coverage Diff             @@
##           master    #3434      +/-   ##
==========================================
- Coverage   83.27%   83.24%   -0.04%     
==========================================
  Files         365      365              
  Lines       64305    64276      -29     
==========================================
- Hits        53549    53505      -44     
- Misses      10756    10771      +15

[VesnaT]

Until this PR the widget expected self.effective_variables are always set when self.data is not None.
With this change, if there are not continuous variables in input data, self.effective_variables happens to be [None, None], which causes several errors.
Maybe you could add a check in check_data() (clearing the plot and showing an error message would be nice as well).

        if self.data is not None:
            if not self.data.domain.has_continuous_attributes(True, True):
            	self.Error.no_continuous_vars()
                self.data = None

I don't mind any other solution, just make sure (a potential test - that should probably be added) passes:

    def test_change_data(self):
        self.send_signal(self.widget.Inputs.data, self.data)
        self.send_signal(self.widget.Inputs.data, Table("titanic"))

[janezd]

Thanks for the fix and the test (I also added a few checks).