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 doing this! Some nitpicking on this graph: https://i.ibb.co/wLd1vSg/donations-income-scatter.png (donations and income)
1) the trendline looks a bit weird. Did you force it to go through (0,0)?
2) Your axis labels initially go up by factors of 100, then the last one only a factor of 10.
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.