Could I get confidence intervals of an function without fit?

for example:

TF1 * f1 = new TF1(“f1”, “[0]+[1]*x”);

f1->SetParameters(0,3);
f1->SetParErrors(1,1);

how should I get and draw the confidence intervals like
(TVirtualFitter::GetFitter())->GetConfidenceIntervals(gint);

?

I’m not sure I understand the meaning of “confidence interval” without a prior fit - can you explain which confidence you’re referring to?

If I have points with TGraphErrors and Fit with function
I could use (TVirtualFitter::GetFitter())->GetConfidenceIntervals(gint) to got the GetConfidenceIntervals at difference points.

but If I only have function with parameters and parameters’error, I don’t find an function to get the Confidence Interval(for example, one sigma) at some points. though I could calculate it by hand using .

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