Naming the axis subdivisions

I have an excel chart which horizontal axis is subdivided. How can I give each subdivision a name/title, instead of just giving the axis a general title?

For example, my graph consists of the number of products that I buy in the supermarket, which I have subclassified
Dairy: yogurt and milk
Meats: chicken, beef and pork
Vegetables: grains and tubers

I wish subheadings "Specific product" and "Category" to appear next to their corresponding axis, not below. 

If it was just a graph, I could do it manually. But the reality is that there are many graphics so it should be done through a configuration.