RGB Parade: what is the variable on the horizontal axis?

Thanks for the replies, and the suggestion of tutorials, but I'm a reasonably capable user of the RGB Parade. I use it in conjunction with RGB curves to accomplish colour correction, similar to what I do in Photoshop – but it's not as easy in Premiere. Certain features available in PS are lacking.

I've never needed to know how the horizontal axis is calibrated, but today I thought I should look into it. And none of the dozen or so web sites that I visited told me what's along the horizontal axis.

Photoshop's Histogram

Let me go to a similar graph that I understand – Photoshop's histogram. The horizontal axis is the level (0-255), while the vertical axis is the number of pixels at each level. So for a dark image, you get a hump at the left, for example. Or you might get a bell curve for a well-exposed image with a good spread in the tonal range.

Premiere's RGB Parade

The vertical axis on the RGB Parade is the level. I used to think it was 0-255, but for certain clips that go right to the bottom (black), I am able to raise them and see a jagged bottom edge, which says to me the RGB Parade is not showing down to 0 on the vertical axis, but to a higher level, say 16. So the graph is being clipped at 16, which means when I brighten the clip, the levels below 16 become visible. At least that's what appears to happen.

So… the vertical axis shows the level (black at the bottom, white at the top) – but what is the horizontal axis showing? What is the variable along the horizontal axis?