Why require valid numbers in every field?

That prevents empty cells or malformed text from being silently treated as zeros.

What if all x-values are the same?

The regression line is undefined because the denominator becomes zero, so the calculator rejects that case.

Does the tool accept decimals?

Yes. Each coordinate can be any finite real number, including scientific notation.

Can I use more than two points?

Yes. The fit is computed from all of the points you enter.