[FIX] Scatter Plot: Always setup plot

[VesnaT]

Issue

When inputing features and data into Scatterplot at the same time handleNewSignals() is invoked only once (handleNewSignals() then calls setup_plot()). Therefore attr_x and attr_y, set from settings, could be the same as input features and self.setup_plot() would never be called.

Description of changes

Always call self.attr_changed() when receiving Features on the input.

Includes

• Code changes
• Tests
• Documentation

[janezd]

Why do we need to call setup_plot when feature input does not actually change the current attr_x and attr_y? Does it have any useful side effects or does it just trigger an unnecessary update?

[janezd]

Answer to self after getting an answer from @VesnaT: if this input features signal is present, super().handleNewSignals() is not called because it's in the else part. The fix is rather dangerous though, because super().handleNewSignals() is still not called; it works just because attr_changed() calls all the same methods that are called by handleNewSignals(), but this can change in the future.

We can merge this PR out of urgency, but this has to be fixed by setting __invalidate to True instead.