It looks like there are waves beetween the points, but it's straight.

<?php
$maxwert = 300;
$size = 10;
$img = imagecreatetruecolor($maxwert, $maxwert);
imagecolorallocate($img, 0, 0, 0);

   for($y=0;$y<$maxwert;$y += $size){
     for($x=0;$x<$maxwert;$x+=$size){
      $r = rand(0,255);
      $g = rand(0,255);
      $b = rand(0,255);
      $color = imagecolorallocate($img, $r, $g, $b);
      
      imagefilledrectangle ($img, $x, $y, $x+$size ,$y+$size, $color);
     
     }
   }                  
header("Content-type: image/png"); 
imagepng($img);
?>