Arduino FFT Spectrum analyzer on VFD display GP1287

#include "U8g2lib.h"
#include "fix_fft.h"
#include <SPI.h>
int buttonPin = 2; 
int oldButtonVal = 0;
// u8g2 set up, bar, line position L & R
#define LINEY 40
#define LINEXL 0
#define LINEXR 256

#define SAMPLES 128

#define AUDIO A0
 U8G2_GP1287AI_256X50_1_4W_SW_SPI u8g2(U8G2_R0, /* clock=*/ 13, /* data=*/ 11, /* cs=*/ 10, /* dc=*/ 9, /* reset=*/ 8);
//U8GLIB_ST7920_128X64_1X lcd(EN, RW, CS); // serial use, PSB = GND

char im[SAMPLES];
char data[SAMPLES];
int barht[SAMPLES];
int nPatterns = 2;
int lightPattern = 1;
void setup()
{
  u8g2.begin(); // inti u8g2
  u8g2.setContrast(25); 
  pinMode(2, INPUT_PULLUP);
}

void loop()
{
  static int i, j;
  int val;
  

  // get audio data
  for(i = 0; i < SAMPLES; i++)
  {
    val = analogRead(AUDIO)* 50; // 0-1023
    data[i] = (char)(val/4 - 128); // store as char
    im[i] = 0; // init all as 0
  }


  // run FFT
  fix_fft(data, im, 7, 0);

  // extract absolute value of data only, for 64 results
  for(i = 0; i < SAMPLES/2; i++)
  {
    barht[i] = (int)sqrt(data[i] * data[i] + im[i] * im[i]);
  }
  
  for(i = 0, j = 0; i < SAMPLES/2; i++, j += 2)
  {
    barht[i] = barht[j] + barht[j + 1];
  }
  
  // u8g2 barchart
  barchart(SAMPLES/4, barht); // plot SAMPLES / 4 = 32 as barchart gen cannot handle 128 bars
} 

// plot line and bar at position and height
void barchart(int n, int bh[])
{
  int i, s, w; // bars, spacing and width
  
  s = (LINEXR - LINEXL) / n;
  int buttonVal = digitalRead(buttonPin);
  if (buttonVal == LOW && oldButtonVal == HIGH) {// button has just been pressed
    lightPattern = lightPattern + 1;
  }
  if (lightPattern > nPatterns) lightPattern = 1;
  oldButtonVal = buttonVal;
  
  switch(lightPattern) {
    case 1:
     w = s / 2;
      break;
    case 2:
     w = s / 1;
  }
  
  u8g2.firstPage();
  do
  {
    u8g2.setFont(u8g2_font_6x12_tf);
    u8g2.drawStr(80, 7, "FFT Audio Spectrum");
    u8g2.drawLine(LINEXL, LINEY, LINEXR, LINEY);
    u8g2.drawStr(0, LINEY + 10, "0");
    u8g2.drawStr(50, LINEY + 10, "1k");
    u8g2.drawStr(110, LINEY + 10, "2k");
    u8g2.drawStr(164, LINEY + 10, "3k");
    u8g2.drawStr(220, LINEY + 10, "4k");
    u8g2.drawStr(240, LINEY + 10, "Hz");
    for(i = 0; i < n; i++)
    {
      u8g2.drawBox(LINEXL + s * i, LINEY - bh[i], w, bh[i] + 1); // u8glib doesn't accept box height of 0
    }
      
  }while(u8g2.nextPage()); 
}