Check this.
- Graph needs to have preferred size.
- Don't override paint, but do paintComponent
(nothing wrong with your example)

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.GridLayout;

import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class TestSlices {

    private static Color    colors[]    = { Color.GREEN, Color.RED, Color.CYAN,
            Color.ORANGE, Color.MAGENTA, Color.BLUE };

    public static void main(String[] args) {
        JPanel pnl = new JPanel(new GridLayout(1, colors.length));
        for (int i = 0; i < 7; i++) {
            pnl.add(new Graph());
        }

        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(pnl);
        f.pack();
        f.setVisible(true);

    }

    static class Graph extends JComponent {

        Graph() {
            setPreferredSize(new Dimension(100, 100));
        }

        protected void paintComponent(Graphics g) {
            // public void paint(Graphics g) {
            int x = 0;
            int y = 0;
            int w = getWidth() / 2;
            int h = getHeight() / 2;
            int shift = 60;
            for (int i = 0, angle = 0; i < colors.length; i++, angle += shift) {
                g.setColor(colors[i]);
                g.fillArc(x, y, w, h, angle, shift);
            }
        }
    }
}