Trying to understand simple example

Greetings to all!

I am a very new GNU Radio user.

For now I am trying to get simple example with spectrum analyzer.

I am encountering problems with understanding the result of the results
I get. I am attaching to screens - one for the schematic and one for Qt
FFT block result.

As for now I do not understand - why do I get some strange -15dB value
on spectrum for I am using a simple gen with 1 volt amplitude. And how
to customize the y-scale correctly.

I hope if anybody can help me with this and clarify those questions.

Thank everyone in advance.

Alex