This is about as simple as I can do for a midi keyboard monitor. It's fixed as an 88 key piano layout. The two variables lo and hi set the bounds of the keyboard display. Potentially these could be sliders. Moreover this listens to all channels so that e.g. a noteon on channel 1 at pitch p will be ended by a note of pitch p on channel 2. This could be fixed by having a larger array of note values: one per channel but that is overkill for what I need, which is simply a display of what key I'm pressing.

desc: jda midi monitor

@init
// note range
lo = 21;
hi = 21+87;

// look up table for colour of note
notes = 100;
colours = 10;
colours[0] = 0;
colours[1] = 1;
colours[2] = 0;
colours[3] = 1;
colours[4] = 0;
colours[5] = 0;
colours[6] = 1;
colours[7] = 0;
colours[8] = 1;
colours[9] = 0;
colours[10] = 1;
colours[11] = 0;

function handleNoteOff()
(
  notes[msg2] = 0;
);

function handleNoteOn()
(
  ( msg3 == 0 ) ? (
    handleNoteOff();
  ) : (
    notes[msg2] = 1;
  );
);

// expand range if high or low notes are black
( colors[lo%12] == 1 ) ? (
  lo -= 1;
);
( colors[hi%12] == 1 ) ? (
  hi += 1;
);
nn = hi-lo+1; // number of notes
nwn = 0; // number of white notes

// count white notes (used to compute width of white note)
i = lo;
loop(nn,
  ( colours[i%12] == 0 ) ? (
    nwn += 1;
  );
  i += 1;
);


@block
while(midirecv(offset,msg1,msg2,msg3)) (
  channel = msg1 & 0xF;
  status = msg1 >> 4;
  ( status == 0x09 ) ? (
    handleNoteOn();
  ) : ( status == 0x08 ) ? (
    handleNoteOff();
  );
  midisend(offset,msg1,msg2,msg3); // pass all through
);

@gfx 880 100 

// white background
gfx_set(255,255,255);
gfx_rect(0,0,gfx_w,gfx_h);

// compute sizes of notes
note_w = gfx_w / nwn;
note_bw = note_w/1.6;
note_bw2 = note_bw/2;
note_h = gfx_h;
note_hb = note_h * 0.6;

// first pass, draw the white notes
i = 0;
note_r = 0;
loop(nn,
  j = i + lo;
  s = notes[j];
  pc = j % 12;
  col = colours[pc];
  pos = poses[pc];

  // set colour
  ( s == 0 ) ? (
    ( j == 60 ) ? ( // middle C light yellow
      gfx_set(1,1,0.4);
    ) : (
      gfx_set(1,1,1);
    );
  ) : (
    ( j == 60 ) ? (
      gfx_set(1,0.25,0);
    ) : (
      gfx_set(1,0,0);
    );
  );
  ( col == 0 ) ? (
    gfx_rect(note_r,0,note_w,note_h);
    note_r += note_w;
  );
  i += 1;
);

// middle pass
// draw the separators between the white notes
note_r = 0;
i = 0;
loop(nn,
  s = notes[i];
  j = i + lo;
  pc = j % 12;
  col = colours[pc];
  pos = poses[pc];

  // draw line between white notes
  ( col == 0 ) ? (
    gfx_set(0,0,0);
    gfx_line(note_r,0,note_r,note_h);
  );

  ( col == 0 ) ? (
    //gfx_rect(note_r,0,note_w,note_h);
    note_r += note_w;
  );
  i += 1;
);

// third pass, draw the black notes
note_r = 0;
i = 0;
loop(nn,
  j = i + lo;
  s = notes[j];
  pc = j % 12;
  col = colours[pc];
  pos = poses[pc];

  // set colour
  ( s == 0 ) ? (
    gfx_set(0,0,0);
  ) : (
    gfx_set(0,0.7,0);
  );
  ( col == 1 ) ? (
    gfx_rect(note_r-note_bw2,0,note_bw,note_hb);
  ) : (
    //gfx_rect(note_r,0,note_w,note_h);
    note_r += note_w;
  );
  i += 1;
);

V2

This one allows you to click on notes to send MIDI.

desc: jda midi key monitor 2
// idea is to allow clicks to be notes

@init
// note range
lo = 21;
hi = 21+87;

// look up table for colour of note
notes = 100;
colours = 10;
colours[0] = 0;
colours[1] = 1;
colours[2] = 0;
colours[3] = 1;
colours[4] = 0;
colours[5] = 0;
colours[6] = 1;
colours[7] = 0;
colours[8] = 1;
colours[9] = 0;
colours[10] = 1;
colours[11] = 0;

playing = 300;
function allPlayingOff()
(
  i = 0;
  while(i<128)
    ( 
      midisend(0,0x80,i,0); // note off on release
      i += 1;
    );
);

function handleNoteOff()
(
  notes[msg2] = 0;
);

function handleNoteOn()
(
  ( msg3 == 0 ) ? (
    handleNoteOff();
  ) : (
    notes[msg2] = 1;
  );
);

// expand range if high or low notes are black
( colors[lo%12] == 1 ) ? (
  lo -= 1;
);
( colors[hi%12] == 1 ) ? (
  hi += 1;
);
nn = hi-lo+1; // number of notes
nwn = 0; // number of white notes

// count white notes (used to compute width of white note)
i = lo;
loop(nn,
  ( colours[i%12] == 0 ) ? (
    nwn += 1;
  );
  i += 1;
);

mouse_play = -1;
mouse_over = -1;
mouse_down = 0;

@block
while(midirecv(offset,msg1,msg2,msg3)) (
  channel = msg1 & 0xF;
  status = msg1 >> 4;
  ( status == 0x09 ) ? (
    handleNoteOn();
  ) : ( status == 0x08 ) ? (
    handleNoteOff();
  );
  midisend(offset,msg1,msg2,msg3); // pass all through
);

( mouse_cap == 0 ) ? (
  ( mouse_play >= 0 ) ? (
    midisend(0,0x80,mouse_play,0); // note off on release
    mouse_play = -1;
  );
  mouse_down = 0;
);

( mouse_cap == 1 ) ? (
  ( mouse_down == 0 ) ? (
    mouse_down = 1;
    ( mouse_play >= 0 ) ? (
      midisend(0,0x80,mouse_play,0); // note off on release
    );
    ( mouse_over >= 0 ) ? (
      mouse_play = mouse_over;
      midisend(0,0x90,mouse_over,80); // note off on release
    );
  );
);

@gfx 880 100 

// white background
gfx_set(255,255,255);
gfx_rect(0,0,gfx_w,gfx_h);

// compute sizes of notes
note_w = gfx_w / nwn;
note_bw = note_w/1.6;
note_bw2 = note_bw/2;
note_h = gfx_h;
note_hb = note_h * 0.6;


// first pass, draw the white notes
i = 0;
note_r = 0;
loop(nn,
  j = i + lo;
  s = notes[j];
  pc = j % 12;
  col = colours[pc];
  pos = poses[pc];

  // set colour
  ( s == 0 ) ? (
    ( j == 60 ) ? ( // middle C light yellow
      gfx_set(1,1,0.4);
    ) : (
      gfx_set(1,1,1);
    );
  ) : (
    ( j == 60 ) ? (
      gfx_set(1,0.25,0);
    ) : (
      gfx_set(1,0,0);
    );
  );
  ( col == 0 ) ? (
    gfx_rect(note_r,0,note_w,note_h);
    ( ( mouse_x >= note_r && mouse_x <= note_r + note_w) &&
      ( mouse_y >= 0 && mouse_y <= note_h ) ) ? (
      mouse_over = j;
    );
    note_r += note_w;
  );
  i += 1;
);

// middle pass
// draw the separators between the white notes
note_r = 0;
i = 0;
loop(nn,
  s = notes[i];
  j = i + lo;
  pc = j % 12;
  col = colours[pc];
  pos = poses[pc];

  // draw line between white notes
  ( col == 0 ) ? (
    gfx_set(0,0,0);
    gfx_line(note_r,0,note_r,note_h);
  );

  ( col == 0 ) ? (
    //gfx_rect(note_r,0,note_w,note_h);
    note_r += note_w;
  );
  i += 1;
);

// third pass, draw the black notes
note_r = 0;
i = 0;
loop(nn,
  j = i + lo;
  s = notes[j];
  pc = j % 12;
  col = colours[pc];
  pos = poses[pc];

  // set colour
  ( s == 0 ) ? (
    gfx_set(0,0,0);
  ) : (
    gfx_set(0,0.7,0);
  );
  ( col == 1 ) ? (
    gfx_rect(note_r-note_bw2,0,note_bw,note_hb);
    ( ( mouse_x >= ( note_r - note_bw2 ) && 
        mouse_x <= ( note_r + note_bw ) ) &&
      ( mouse_y >= 0 && mouse_y <= note_hb ) ) ? (
      mouse_over = j;
    );
  ) : (
    //gfx_rect(note_r,0,note_w,note_h);
    note_r += note_w;
  );
  i += 1;
);