C Plotting Program

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <grx20.h>
#include <grxkeys.h>

int data_count, i;
float sum_y, sum_x, sum_x_squared, sum_xy=0;
float gradient, y_intercept, *x_values, *y_values;
float x[100],y[100];
FILE *store;

void file_save(void) {
     store = fopen("j:/StoredValues.txt", "w");
     for (i=0; i<data_count; i++) {
     fprintf ( store, "%f,%f\n", x_values[i], y_values[i]);
     }
     fclose(store);
}

void file_read(void) {
     store = fopen ("j:/StoredValues.txt", "r");
     if (store == NULL )
              printf("\nError: Failed To Open Previous Data File - Program Will Continue Anyway\n");
     else {
              printf("\nSuccess: Data From Previous Run Imported\n");
              i=0;
              do {
              fscanf ( store, "%f,%f\n", &x[i], &y[i]);
              x_values = x;
              y_values = y;
              printf("%12f%12f\n", x_values[i], y_values[i]);
              i=i+1;
              } while (!feof(store));
              fclose(store);
     }
}

void calculations(void) {
     for (i=0; i<data_count; i++) {
         sum_x = sum_x + x_values[i];
         sum_x_squared = sum_x_squared + (x_values[i]*x_values[i]);
         sum_y = sum_y + y_values[i];
         sum_xy = sum_xy + (y_values[i]*x_values[i]);
     }
     if (data_count==1) { /*Prevents Attempting To Calculate A Gradient Of A Single Point*/
         gradient=0;
         y_intercept=0;
     }
     else {
     gradient = (data_count*sum_xy - sum_x*sum_y)/(data_count*sum_x_squared - (sum_x*sum_x));
     y_intercept = (sum_y - gradient*sum_x)/data_count;
     }
}

int main()
{
    int menu;

    printf("PROGRAM TO DRAW LINE OF BEST FIT\n");

    menu = 0;
    file_read();
    while (menu != 4) {
          printf("\n1. Enter Data Points\n");
          printf("2. View Data Points\n");
          printf("3. Create Graph From Data\n");
          printf("4. Exit Program\n");
          printf("\nEnter An Option: ");
          scanf("%d", &menu);

          switch(menu) {
          case 1:
               {
               printf("\nPlease Enter How Many Data Points You Wish To Enter: \n");
               scanf("%d", &data_count);
               x_values=(float*)calloc(data_count,sizeof(*x_values)); /*Allows for dynamicly sized arrays*/
               y_values=(float*)calloc(data_count,sizeof(*y_values));
               if (x_values==NULL) {
                   printf("Error! Memory Could Not Be Allocated. ");
                   exit(0);
               }
               printf("\nPlease Enter Your X Values: \n");
               for(i=0;i<data_count;i++) {
               printf("\nx<%d>: ", i);
               scanf("%f", x_values+i);
               }
               printf("\nPlese Enter Your Y Values: \n");
               for(i=0;i<data_count;i++) {
               printf("\ny<%d>: ", i);
               scanf("%f",y_values+i);
               }
               calculations();
               file_save();
               break;
               }

          case 2:
               {
               if (x_values==NULL) {
               printf("\n\nError! Please Enter Data First\n\n");
               break;
               }
               printf("\nEntered Values: \n");
               printf("\n%12s%12s\n\n", "x", "y");
               for(i=0;i<data_count;i++) {
               printf("%12f%12f\n", x_values[i], y_values[i]);
               }
               printf("\nSum Of X Values: %f\n", sum_x);
               printf("Sum Of Y Values: %f\n", sum_y);
               printf("Gradient Of The Line: %f\n", gradient);
               printf("Y Intercept Of The Line: %f\n", y_intercept);
               break;
               }
          case 3:
          case 4:
               default:
                 {
                   printf("\n\nERROR invalid\n\n");
                 }
                 }
                 }

    free(x_values);
    free(y_values);
    return 0;
}