Matrix assigns a letter to each risk. This is used for navigating the graphs.
No, you cannot set this to numeric. Matrix only uses letters. When you are viewing the heatmap, you can use the letter of the item to filter out the other items from the graph. You can also press it a second time to see the scatter plot.
If you have more than 26 risks in your list, Matrix will add subsequent alphabets. For example item 27 would be labelled as A2 and items 54 would be B3.
For more information about navigating the additional alphabets click here.