How to use TF12?

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ROOT Version: 6.40.04
Platform: macosxarm64
Compiler: Apple clang version 21.0.0 (clang-2100.1.1.101) std202002

I am trying to use TF12 to create projections of a TF2, and it doesn’t seem to work as documented. I have made a very simple example which I have posted below. I have a simple 2D function with 2 parameters. I want to make one projection, change a parameter, and make another projection. The documentation says you have to use DrawCopy to accomplish this task. This doesn’t seem to work. Also the title of the graph says “x=2” even though it should say “y=2” but that is a minor annoyance. I don’t understand why I don’t see a second line in the canvas. Any help instructing me in the ways of TF12 will be appreciated!

To run the example: Load (.L) this code into root and then “plot_func()”.

double my_func(double *args, double *parms){
    double x = args[0];
    double y = args[1];
    double a = parms[0];
    double b = parms[1];

    return a*x + b*y*y;

}

void plot_func(){

    TF2 *my_tf2 = new TF2("my_tf2", my_func, 0., 2., 0., 4., 2);

    my_tf2->SetParameter(0,1.0);
    my_tf2->SetParameter(1,2.0);

    TF12 *my_tf12_x = new TF12("my_tf12_x", my_tf2, 2., "x");

    auto *can = new TCanvas();

    my_tf12_x->SetLineColor(1);
    my_tf12_x->DrawCopy("f");

    my_tf2->SetParameter(0,0.5);
    my_tf12_x->SetLineColor(3);
    my_tf12_x->DrawCopy("same");

}
```