During exploratory data analysis on a Spark DataFrame, a data scientist examines the relationship between a house's 'age' and its 'maintenance_cost'. A scatter plot shows that maintenance cost consistently increases as age increases, but the growth is clearly curved rather than a straight line. The scientist computes the Pearson correlation coefficient and gets a value of 0.61, which seems to understate how strong the relationship looks visually. Which action will best quantify the strength of this monotonic but nonlinear relationship?