code: def plot_single(self, col, ax):
      self._mean[col].plot(ax=ax, label="mean")
      ax.set_xlabel("")
      ax.set_ylabel("mean")
      ax.legend()
  
      right_ax = ax.twinx()
  
      self._std[col].plot(ax=right_ax, label="std", color="green")
      right_ax.set_xlabel("")
      right_ax.set_ylabel("std")
  
      h1, l1 = ax.get_legend_handles_labels()
      h2, l2 = right_ax.get_legend_handles_labels()
  
      ax.legend().set_visible(False)
      right_ax.legend(h1 + h2, l1 + l2)
      ax.set_title(col)
  
imports: import pandas
  import numpy
  from qlib.contrib.report.data.base import FeaAnalyser
  from qlib.contrib.report.utils import sub_fig_generator
  from qlib.utils.paral import datetime_groupby_apply
  from qlib.contrib.eva.alpha import pred_autocorr_all
  from loguru import logger
  import seaborn
label_desc: Use image node_id 2 for calling the FeaMeanStd obj's underlying member method code with example usage: obj.plot_single(col, ax) without return types