I've been reading about ECDSA in here.
Step 11 says that
you only need the ‘x‘ value (20 bytes) for the signature, and that value will be called ‘R‘
Does that say that R is the x-coordinate of the public key. But I don't see that happening in a raw transactions.
For the pizza transaction, it's scriptSig looks like that：
The first one is signature,the second one is the public key. I don't see any relationship between the two items.