Here is a Bare Bones Arduino clone - with nothing attached to the Analog pins. The signal here on a Bare Bones board is more "clean" than my old arduino duemilanove.

Here I'm just reading another pin at the same time ... you wouldn't expect that to change the voltage level BUT IT DOES  !!!!

Here I just stuck a wire in A2 - look what changes happen with just a wire ! Wire is like a GIANT amplifier !

with a regular jumper wire like this.

And now with some Aluminum Foil wrapped around the end of the wire EVEN BIGGER AMPLIFIER !


Nice, I suspect you have a very nearly flat line because :

1. you have a good Arduino board

2. you dont have a lot of crazy electrical interference going on everywhere, like bad flourecent lights, or unsheilded power supplies nearby, or any LOUD stuff around me :)