How to label a risk diagram in an understandable way?

I am improving a risk diagram. It seems difficult to place the axis labels.

Here is the current situation:

My customers and I are not happy with this. Customers suggest to have the axis labels vertically and horizontally. This is where I don't find a better solution. Having the labels near the diagram seems to rip the relation between the value label and the picture. Also, the meaning of "vertically aligned font" and "horizontally aligned font" are mixing up.

If I take the axis labels outside, then they are some strangeres far away, my eye is not relating them to the color field:

What further options are there in designing such a diagram?