Hello Martin,
The spectrum graph by default keeps the left hand of the graph at 0 Hz. If you want some other position, use the mouse, and grab the graph and drag it how you want. You can then zoom in and out on the new center point until the 0 Hz axis is visible again, in which case the left hand side again becomes locked to 0 Hz.

Alternatively you can use the keyboard shortcuts to zoom directly on a point, and the 0 Hz will automatically unlock - position the tracer where you want to zoom, and then use Ctrl Left/Right Arrow to zoom on that point.

We have done it this way because if you just have it free zooming it becomes very annoying trying to get the 0 Hz point to stay at the left hand side as you zoom back out.