Erasure recovery knows which shard is missing, but not its value.
One shard is erased
The missing shard is the value at x equals 1. The table still shows it so the lesson can be checked.
missing x=1
Keep two surviving points
The recovery uses kept point x 0 with value 2, and x 2 with value 3.
(0,2),(2,3)
Two points rebuild a line
A degree-one shard rule is a line, so two kept points are enough in this tiny example.
two kept points rebuild the line
Summary
The recovery ignores the erased value and rebuilds it from the kept points.
erasure means location known, value missing