_client(synthesis_xs[i])+5.5, y_to_client(synthesis_ys[i])+5.5);
        synthesis_context.lineTo(x_to_client(synthesis_xs[i])-5.5, y_to_client(synthesis_ys[i])-5.5);
    }
    synthesis_context.strokeStyle="#AA3311";
    synthesis_context.stroke();
    synthesis_context.closePath();

    // tangents
    synthesis_context.beginPath();
    synthesis_context.setLineDash([4, 4]);
    synthesis_context.moveTo(x_to_client(synthesis_xs[0]), y_to_client(synthesis_ys[0]));
    synthesis_context.lineTo(x_to_client(s