Use the zorder kwarg where the lower the zorder the further back the plot, e.g.
plt.plot(series1_x, series1_y, zorder=1)
plt.plot(series2_x, series2_y, zorder=2)
plt.scatter(series2_x, series2_y, zorder=3)
Use the zorder kwarg where the lower the zorder the further back the plot, e.g.
plt.plot(series1_x, series1_y, zorder=1)
plt.plot(series2_x, series2_y, zorder=2)
plt.scatter(series2_x, series2_y, zorder=3)