Canvas canvas = new Canvas(shell, SWT.NONE);
		canvas.addListener (SWT.Paint, e ->  {
	        	GC gc = new GC(canvas);
	        canvas.setBackground(shell.getDisplay().getSystemColor(SWT.COLOR_WHITE));
	        	int maxX = canvas.getSize().x; int maxY = canvas.getSize().y;
	     	    int halfX = (int) maxX / 2; int halfY = (int) maxY / 2;
	        	gc.setForeground(display.getSystemColor(SWT.COLOR_BLACK));
	        	gc.setLineStyle(SWT.LINE_SOLID);
	    	    gc.drawLine(0, halfY, maxX, halfY);
	    	    gc.drawLine(halfX, 0, halfX, maxY);
	       
	    });
......
 Button b = new Button(composite, SWT.PUSH); b.setText("Plot");
	    b.pack();
	    b.addListener(SWT.Selection, e-> {
	    	GC gc = new GC(canvas);
	        	  gc.setForeground(display.getSystemColor(SWT.COLOR_RED));
		    	    gc.drawOval(100, 200, 20, 20);
	     });
//And nothing happens!
	    canvas.redraw();