It looks like the example demo_parasite_axes2 lost its “Temperature” second y-axis in the devdocs gallery. See the non-devodcs version for a comparison.

For those who do not like to click, here are the two PNGs with default dpi:

• devdocs version
  
• non devdocs version
  

NB: a diff between the code source of each example results in an empty file.

Edit: “twin y-axis” ← “second y-axis”, which should be more correct.