As I need to motivate it in an essay, I would like to have a measure of the amount of linearity and I used the Pearson product-moment correlation coefficient, obtaining a value of -0.1559585. But after having tested the normality of the variables with a Shapiro-Wilk test, I have obtained that that the X values are not normally distributed, therefore I cannot use the Pearson coefficient to do that. I read that I could computed Spearman's rank correlation coefficient as the X values don't follow a normal distribution, but this coefficient gives an estimation of the monotonic association between X and Y, while I would like to have a quantification of the linearity between X and Y. Do you know how I can compute a quantity that express this, please?
Thank you very much.
Edit: The qqplot of X is the following