GLM fitting, testing and graphics plotting

Hello,

I would be very grateful if you could help me with some doubts regarding GLM. My data has two groups (controls and cases) and I want to estimate how the gray matter volume is related to days of drug abstinence, after correcting for age, in both groups (controls, cases). I imagine this like two fitting lines (one for controls, one for cases) y-axis: gray matter, x-axes:days of abstinence (but corrected for age) and testing the slopes.

I have started this simple code, but I dont't know how to code the groups (0, 1 or 1 - 1) or whether I should demean age and abstinence. Also, I would like to plot a graphic with two lines to see how the data is fitted (after correcting for age). How could I do that?

GM =[2;5;8;4]; % Gray matter

group=[0;0;1;1]; % group (0:controls, 1:cases)

age=[45;20;15;58]; % age (years old)

days_abst=[5;8;7;6]; % days of abstinence (behavior value to test)

d = dataset(GM,group,age,days_abst);

d.group=ordinal(d.group);

m = GeneralizedLinearModel.fit(d,'GM~group+age+days_abst');

devianceTest(m);

coefTest(m,[0 0 0 1]) %? Test if the relationship between GM and days_abst is %significantly different in controls and cases

Thank you very much,

Ursula