Fragment Shader start template from https://www.shadertoy.com/view/Md23DV

fragment-sharder-start.glsl

//uniform vec2 resolution;

#define PI 3.14159265359
  
//Anti-aliased coordinate system grid
float coordinateGrid(vec2 coords) {
  //These colors are unused and irrelevant in current implementation.
  vec3 axesColor = vec3(0.0, 0.0, 1.0);
  float axesThickness = 0.015;

  vec3 gridColor = vec3(0.5);
  float gridThickness = 0.008;

  float retrn = 0.0; //

  //Draw grid lines
  const float tickWidth = 0.1;
  for(float i=-2.0; i<2.0; i+=tickWidth) {
    // "i" is the line coordinate.
    retrn += 1.0 - smoothstep(0.0, gridThickness, abs(coords.x - i));
    retrn += 1.0 - smoothstep(0.0, gridThickness, abs(coords.y - i));
  }

  //Draw the axes
  retrn += 1.0 - smoothstep(0.001, axesThickness, abs(coords.x));
  retrn += 1.0 - smoothstep(0.001, axesThickness, abs(coords.y));

  return retrn;
}

//returns 1.0 if inside circle
float circle(vec2 coords, vec2 center, float radius) {
  	float antiAliasTolerance = 0.005;
	return 1.0 - smoothstep(radius - antiAliasTolerance, radius + antiAliasTolerance, length(coords - center));
}

void main() {

	vec2 defaultCoords = vec2(gl_FragCoord.xy/resolution.xy);
	vec2 viewportCenterCoords = 2.0 * vec2(gl_FragCoord.xy - resolution.xy/2.0) / resolution.y;
  	
  	vec3 colorOut;

  // --------------------------------------
  // ------------- Start ------------------
  
  	   // Fancy Code Here
  
  // -------------  End  ------------------
  // --------------------------------------
  
  vec3 gridColor = vec3(0.0, 0.634, 0.876);
  colorOut = mix(colorOut, gridColor, coordinateGrid(viewportCenterCoords));
  
  vec3 pixel = colorOut;
  gl_FragColor = vec4(pixel, 1.0);
}

TODO: clean up and needs some polishing.