How to make this number obvious for user to click on it to see list of items

Question

What can be done to this matrix to make it more obvious to end user to click on number to see list of items in it, when a user will click on the box, if there any number shown on it e.g. starting from "1" then it will show a list of items on the right side,

I am going to make it a like a graph.

Answer 1

Add a drop shadow to the box to a elevate it, making it appear tactile.

Or underline the number to indicate it's clickable.

The drop shadow approach is better because it correctly indicates the entire square as clickable, versus just the number.

For example (the drop shadow and underline are poorly done because I'm on mobile right now):

Answer 2

A couple things come to mind like adding a border, bolding the text, but maybe the simplest would be adding a tooltip that explains what you want to have happen when they click.

Answer 3

I think a Chevron with some drop-shadow would be sufficient to let the user know the item is clickable. A border around the object when hovering/clicking may help as well. If it could be viewed on a desktop, I would also add some hover-over tooltips as well.