When there is a chart with two axis (one visible, the second one hidden), the second axis gets visible once you unselect (Click on the data set legend item) a data set.
Check the images below:
- Axis is displayed correctly (Only one axis shown):
2. Unselect one data set (Click on the legend item):
3. Second axis is displayed despite it should be hidden:
After adding the data set again using the legend item, the axis still looks the same (Both shown).
For me, this looks like a small bug with the visible property.