I am currently working on my thesis which is concerned with abnormality of differences between index scores on an intelligence test. An important factor here are the correlations between indexes. There are 4 indexes, hence 6 possible pairs. Correlations between indexes are provided for 13 age bands.

To investigate potential age effects on the correlations (and non-linear relationships) I carried out hierarchical multiple regression using polynomial functions of age (i.e. age_band, age_band_squared, age_band_cubed) as predictor variables and the correlation between a pair of indexes as dependent variable.

Example of regression equation: IndexA*IndexB = 0.2 + 0.13 x age - 0.02 x age_squared + 0.001 x age_cubed.

PROBLEM:

My supervisor now wants me to plot the regression lines of the 6 pairs of indexes all on one figure; with age as x-axis and correlation as y-axis. He doesn't want the actual data points in there but only the regression lines.

I tried to do this with SPSS, but can't work out how to plot different regression lines in the same graph.

Help would be greatly appreciated! Thanks!