The axis was just mislabelled (one missing 0). We updated the graph to fix that.
As to the trendline, we just used a line of best fit, which assumes a linear relationship. The low R^2 (~30%) of this linear Donations~Income regression explains why it “looks a bit weird”. It was used as an easy to interpret visual that depicted a simplified relationship between income and donations but one which demonstrated the correct direction of effect. This does have the disadvantage of being prone to overfitting, and as we noted “there are some large outliers driving this very strong relationship”. We might expect a better fit for a nonlinear relationship, however, the later analysis with differing linear responses for different donor groups, was a reasonable fit.
Thanks for your comment Elizabeth.
The axis was just mislabelled (one missing 0). We updated the graph to fix that.
As to the trendline, we just used a line of best fit, which assumes a linear relationship. The low R^2 (~30%) of this linear Donations~Income regression explains why it “looks a bit weird”. It was used as an easy to interpret visual that depicted a simplified relationship between income and donations but one which demonstrated the correct direction of effect. This does have the disadvantage of being prone to overfitting, and as we noted “there are some large outliers driving this very strong relationship”. We might expect a better fit for a nonlinear relationship, however, the later analysis with differing linear responses for different donor groups, was a reasonable fit.