It's smoothed.

Always wondered what that gray area represents

Elevation.

https://preview.redd.it/2q5phcrean1d1.jpeg?width=828&format=pjpg&auto=webp&s=0a2e7e1a3ad0e8f18f1eb78c50c8b521ec086290 Here’s a recent bike ride I did. You can see the grey part in the speed graph is the same as the elevation graph above it