Electronic – What are Normalized graphs

That's a "log log" graph. You may notice that the major divisions go up by factors of ten.

The minor divisions in the example, say starting after 100, are 200, 300, 400, and so on up to 900.

log(100) is 2

log(1000) is 3

So halfway between is \$10^{2.5}\$ or about 316.

It is said that just about everything is approximately linear if you plot it on a log-log graph with a fat marker.

What is meant by a "normalized series load"

A digital input requires a certain amount of current in order to pull the input stage high or low enough to have a voltage within the logic thresholds. This amount of current is designated "1 normalized series load", and it varies per family.

and why would you need an input buffer?

Certain logic devices, e.g. muxes, have complex structures that consist of more than 1 load. The input buffer hides these extra loads.

How can this number be different for each type of logic level?

Both TTL inputs and TTL push-pull outputs are asymmetrical, but not balanced to each other. This results in a discrepancy in the load handling.

What is meant by ".. facilitate the connection of unused inputs to used inputs"?

This one I'm not sure about. Floating TTL inputs are seen as high regardless, so I'm not sure how this would come into play.