Fig. 4
a Scatter plot and LS mean regression \(\widehat {\mu } _{y|x_{1}}\) (red) of the CO2 emission per capita related to ln(GDP) x 1 when the E.C. is 2980 kilowatts. b Scatter plot and LS mean regression \(\widehat {\mu }_{y|x_{2}}\) (red) of the CO2 emission per capita related to ln(E.C.) x 2 when GDP is $13,359.73