code: def plot_to_base64(ax):
      bytes_data = BytesIO()
      ax.figure.savefig(bytes_data, format="png")
      bytes_data.seek(0)
      b64_data = base64.b64encode(bytes_data.read())
      plt.close(ax.figure)
      return b64_data
  
imports: import base64
  from io import BytesIO
  import unittest
  import numpy
  import pandas
  from pyspark import pandas
  from pyspark.pandas.config import set_option, reset_option
  from pyspark.testing.pandasutils import have_matplotlib, matplotlib_requirement_message, PandasOnSparkTestCase, TestUtils
label_desc: Use image node_id 7 for calling the SeriesPlotMatplotlibTestsMixin obj's underlying member method code with example usage: obj.plot_to_base64(ax) and returns: b64_data