Why I can't plot a graph in this coding?

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C++

#include <stdio.h>
#include <math.h>
#include <chplot.h>
#define N 10         /*for 5 step size*/
#define A 10.0      /*constant value of shear stress in kgf/mm^2*/


double calculate_thickness(double r);

int main(){
    
    int  x0=100,xf=500,xstep=50;  
    double i,j,r,w,E;
    double P[N],t[N];

    printf("Enter the value of plate width, w in mm: ");      /*plate width in mm*/
    scanf("%lf",&w);
    
    printf("*Enter 1 for radiography inspection\n");         /*selection of inspection*/
    printf("*Enter 2 for no radiography inspection\n");
    printf("Enter your type of inspection: ");
    scanf("%lf",&r);
    
    printf("  Tensile load,P(kN)   |    thickness,t(mm) \n");   /*for table uses*/
    printf("--------------------------------------------\n");
   
    if(r == 1){
        E=0.9;  /*efficiency when radiography inspection 90%*/
        for (i=0; i<n;>            P[i]=x0+i*xstep;
            t[i]=P[i]/(w*(E*A));    /*equation to find the thickness of welding specimen*/
            printf("     %6.f            |     %6.3f     \n",P[i],t[i]);
        }
    }
    else if(r ==2){
        E=0.65;  /*efficiency when no radiography inspection 65%*/
        for (j=0; j<n;>            P[j]=x0+j*xstep;
            t[j]=P[j]/(w*(E*A));    /*equation to find the thickness of welding specimen*/
            printf("     %6.f            |     %6.3f     \n",P[j],t[j]);
        }
    }
    else{
        printf("Error!\n");
    }
    return 0;
}
double calculate_thickness(double r)
{
    int i,j,r;
    double t[N],P[N];
    
    if (r == 1){
        scanf("%lf",&P[i]);
        scanf("%lf",&t[i]);
    }
    else if (r == 2){
        scanf("%lf",&P[j]);
        scanf("%lf",&t[j]);
    }
    else{
        printf("Error!");
    }
    plotxy(P, t, N, "Acceleration Plot","Tensile load(kN)", "thickness(mm)");
    return 0;
}

What I have tried:

I already try to used another function and check the note what I have learn in array rules. I have no idea why the graph is not come out. The error said that "ERROR: computational array type qualifier applied to a scalar variable
ERROR: function cannot return C array, it can only return computational array"