Many law departments should go beyond the typical metrics of concentration of spending or convergence of law firms. A law firm usage map helps.
Look at the law firms that were in your top ten of spending for each or any of the past four years. List all the firms that were in the top 10, one per row, and the amounts paid them over the four years. Sort by the names on that list. Create a column for each year. Then, in Excel use colored bars to show each year in which each firm was in the top 10. One or two might have made the top 10 all four years. Another handful will have been in three of the four years, and so forth.
This map of consistent usage, the total spending on which firms may account for half or more of the department’s external spend during those years, graphically shows how consistently the department has turned to its primary law firms.