> the sheer amount of duplication between the tests for both plot and quickplot made it notably hard to avoid duplication errors

Indeed ... I'm having to work to see the wood for the trees. Have you considered writing all the tests with the form:
```python
def test_yaxis_labels(self):
    iplt.points(self.cube, coords=('bar', 'str_coord'))
    self.assertBoundedTicks('yaxis')
```