plotInteraction issue with categorical data...wtf?

after this: fitlm(X,y,'interactions','Categorical',[1],'RobustOpts','on') then the following are not working:

>> plotInteraction(ans,x1,x2) Undefined function or variable 'x1'.

SO

>> ans.CoefficientNames(2)

ans =

    'x1_1'

OK

>> plotInteraction(ans,ans.CoefficientNames(2),ans.CoefficientNames(3)) Error using classreg.regr.FitObject/identifyVar (line 541) 'x1_1' is not a variable for this fit.

Error in LinearModel/plotInteraction (line 62) [vname1,vnum1] = identifyVar(model,var1);

BUT this does work for some odd reason (given that >> ans.Formula.VariableNames(1)

ans =

    'x1'
):
>> plotInteraction(ans,ans.Formula.VariableNames(1),ans.Formula.VariableNames(2))

[Ben]

what a stupid question, loser! thanks, schmuck!

[Tom Lane]

Not so sure this is a stupid question. The plotInteraction function wants to treat X1 as a single term, and does not expect a name like X1_1 which represents the coefficients of just one category.