plt.AddSignal(ScottPlot.DataGen.Sin(51));
plt.AddSignal(ScottPlot.DataGen.Cos(51));

static string customTickFormatter(double position)
{
    if (position == 0)
        return "zero";
    else if (position > 0)
        return $"+{position:F2}";
    else
        return $"({Math.Abs(position):F2})";
}

plt.XAxis.TickLabelFormat(customTickFormatter);
plt.YAxis.TickLabelFormat(customTickFormatter);