Canvas Example
public class GridPenCanvas extends Canvas implements MouseListener, MouseMotionListener{
int old_mouse_x = 0; // old position (x,y) for mouse drawing
int horizontal_spacing = 0; // pixels between rows and columns,
int vertical_spacing = 0;
public GridPenCanvas(int horiz_spacing, int vert_spacing) {
horizontal_spacing = horiz_spacing;
vertical_spacing = vert_spacing;