How to change Y-axis minimum and maximum values

I THOUGHT I knew how to do this. I THOUGHT I had done it dozens of times. But now... the obvious doesn't work.

I created a simple bar chart. Using its "Auto" feature, Keynote chooses a maximum and minimum value to be shown on the Y axis. But I want the Y axis to start at zero and end at, say, 6. I go to the chart inspector, click on the Axis tab, and there are my boxes to enter my own custom Min and Max values for the Y axis. I click in one (just as I've always done), and the word "Auto" disappears, and the blinking insertion point appears.

So far, so normal.

But when I try to type a number into either box, Keynote beeps at me, doesn't let me enter anything, and returns the box to the Auto value.

Why can't I do this? I've done it dozens of times in the past. It's totally consistent with what the documentation says I can do. What could possibly be wrong?

It must be something very simple and obvious, so obvious I can't see it. But what? I've done this before! What could I be getting wrong???