Grid plugin from https://imagej.net/plugins/grid.html

Grid_.java

import ij.*;
import ij.process.*;
import ij.gui.*;
import java.awt.*;
import java.awt.geom.*;
import java.util.*;
import ij.plugin.*;
import ij.measure.*;

public class Grid2_ implements PlugIn, DialogListener {
    private static double crossSize = 0.1;
    private static String[] colors = {"Red","Green","Blue","Magenta","Cyan","Yellow","Orange","Black","White"};
    private static String color = "Cyan";
    private final static int LINES=0, HLINES=1, CROSSES=2, POINTS=3, NONE=4;
    private static String[] types = {"Lines","Horizontal Lines", "Crosses", "Points", "None"};
    private static String type = types[LINES];
    private static double areaPerPoint;
    private static boolean randomOffset, startAtTopLeft;
    private static boolean bold;    
    private Random random = new Random(System.currentTimeMillis());
    private ImagePlus imp;
    private double tileWidth, tileHeight;
    private int xstart, ystart;
    private int linesV, linesH;
    private double pixelWidth=1.0, pixelHeight=1.0;
    private String units = "pixels";

    public void run(String arg) {
        imp = IJ.getImage();
        showDialog();
    }
        
    void drawPoints() {
        int one = 1;
        int two = 2;
        GeneralPath path = new GeneralPath();
        for(int h=0; h<linesV; h++) {
            for(int v=0; v<linesH; v++) {
                float x = (float)(xstart+h*tileWidth);
                float y = (float)(ystart+v*tileHeight);
                path.moveTo(x-two, y-one); path.lineTo(x-two, y+one);
                path.moveTo(x+two, y-one); path.lineTo(x+two, y+one);
                path.moveTo(x-one, y-two); path.lineTo(x+one, y-two);
                path.moveTo(x-one, y+two); path.lineTo(x+one, y+two);
            }
        }
        showGrid(path);
    }

    void drawCrosses() {
        GeneralPath path = new GeneralPath();
        float arm  = (int)Math.round(crossSize*tileWidth);
        if (arm<3) arm=3;
        for(int h=0; h<linesV; h++) {
            for(int v=0; v<linesH; v++) {
                float x = (float)(xstart+h*tileWidth);
                float y = (float)(ystart+v*tileHeight);
                path.moveTo(x-arm, y);
                path.lineTo(x+arm, y);
                path.moveTo(x, y-arm);
                path.lineTo(x, y+arm);
            }
        }
        showGrid(path);
    }

    void showGrid(Shape shape) {
        if (shape==null)
            imp.setOverlay(null);
        else {
            Roi roi = new ShapeRoi(shape);
            roi.setStrokeColor(getColor());
            if (bold && linesV*linesH<5000) {
                ImageCanvas ic = imp.getCanvas();
                double mag = ic!=null?ic.getMagnification():1.0;
                double width = 2.0;
                if (mag<1.0)
                    width = width/mag;
                roi.setStrokeWidth(width);
            }
            imp.setOverlay(new Overlay(roi));
        }
    }

    void drawLines() {
        GeneralPath path = new GeneralPath();
        int width = imp.getWidth();
        int height = imp.getHeight();
        for(int i=0; i<linesV; i++) {
            float xoff = (float)(xstart+i*tileWidth);
            path.moveTo(xoff,0f);
            path.lineTo(xoff, height);
        }
        for(int i=0; i<linesH; i++) {
            float yoff = (float)(ystart+i*tileHeight);
            path.moveTo(0f, yoff);
            path.lineTo(width, yoff);
        }
        showGrid(path);
    }

    void drawHorizontalLines() {
        GeneralPath path = new GeneralPath();
        int width = imp.getWidth();
        int height = imp.getHeight();
        for(int i=0; i<linesH; i++) {
            float yoff = (float)(ystart+i*tileHeight);
            path.moveTo(0f, yoff);
            path.lineTo(width, yoff);
        }
        showGrid(path);
    }

    void showDialog() {
        int width = imp.getWidth();
        int height = imp.getHeight();
        Calibration cal = imp.getCalibration();
        int places;
        if (cal.scaled()) {
            pixelWidth = cal.pixelWidth;
            pixelHeight = cal.pixelHeight;
            units = cal.getUnits();
            places = 2;
        } else {
            pixelWidth = 1.0;
            pixelHeight = 1.0;
            units = "pixels";
            places = 0;
        }
        if (areaPerPoint==0.0)
            areaPerPoint = (width*cal.pixelWidth*height*cal.pixelHeight)/81.0; // default to 9x9 grid
        GenericDialog gd = new GenericDialog("Grid...");
        gd.addChoice("Grid type:", types, type);
        gd.addNumericField("Area per point:", areaPerPoint, places, 6, units+"^2");
        gd.addChoice("Color:", colors, color);
        gd.addCheckbox("Bold", bold);
        gd.addCheckbox("Random offset", randomOffset);
        gd.addCheckbox("Start at coordinate 0,0", startAtTopLeft);
        gd.addDialogListener(this);
        dialogItemChanged(gd, null);
        gd.showDialog();
        if (gd.wasCanceled()) 
            showGrid(null);
    }

    public boolean dialogItemChanged(GenericDialog gd, AWTEvent e) {
        int width = imp.getWidth();
        int height = imp.getHeight();
        type = gd.getNextChoice();
        areaPerPoint = gd.getNextNumber();
        color = gd.getNextChoice();
        bold = gd.getNextBoolean();
        randomOffset = gd.getNextBoolean();
        startAtTopLeft = gd.getNextBoolean();
        double minArea= (width*height)/50000.0;
        if (type.equals(types[CROSSES])&&minArea<50.0)
            minArea = 50.0;
        else if (minArea<16)
            minArea = 16.0;
        if (areaPerPoint/(pixelWidth*pixelHeight)<minArea) {
            String err = "\"Area per Point\" too small";
            if (gd.wasOKed())
                IJ.error("Grid", err);
            else
                IJ.showStatus(err);
            return true;
        }
        double tileSize = Math.sqrt(areaPerPoint);
        tileWidth = tileSize/pixelWidth;
        tileHeight = tileSize/pixelHeight;
        if (randomOffset) {
            xstart = (int)(random.nextDouble()*tileWidth);
            ystart = (int)(random.nextDouble()*tileHeight);
        } 
        else if(startAtTopLeft){
            xstart = 0;
            ystart = 0;
        }
        
        else {
            xstart = (int)(tileWidth/2.0+0.5);
            ystart = (int)(tileHeight/2.0+0.5);
        }
        linesV = (int)((width-xstart)/tileWidth)+1; 
        linesH = (int)((height-ystart)/tileHeight)+1;
        if (gd.invalidNumber())
            return true;
        showGrid();
            return true;
    }

    private void showGrid() {
        if (type.equals(types[LINES]))
            drawLines();
        else if (type.equals(types[HLINES]))
            drawHorizontalLines();
        else if (type.equals(types[CROSSES]))
            drawCrosses();
        else  if (type.equals(types[POINTS]))
            drawPoints();
        else
            showGrid(null);
    }

    Color getColor() {
        Color c = Color.cyan;
        if (color.equals(colors[0])) c = Color.red;
        else if (color.equals(colors[1])) c = Color.green;
        else if (color.equals(colors[2])) c = Color.blue;
        else if (color.equals(colors[3])) c = Color.magenta;
        else if (color.equals(colors[4])) c = Color.cyan;
        else if (color.equals(colors[5])) c = Color.yellow;
        else if (color.equals(colors[6])) c = Color.orange;
        else if (color.equals(colors[7])) c = Color.black;
        else if (color.equals(colors[8])) c = Color.white;
        return c;
    }
}