docstr: Writes scalar data, starting at step 0.
  
  Args:
    run: string run name.
    tag: string tag name.
    data: list of scalar values to write at each step.
code: def _write_scalar_data(self, run, tag, data=[]):
      """Writes scalar data, starting at step 0.
  
      Args:
        run: string run name.
        tag: string tag name.
        data: list of scalar values to write at each step.
      """
      subdir = os.path.join(self._logdir, run)
      writer = tf.summary.create_file_writer(subdir)
  
      with writer.as_default():
          step = 0
          for datum in data:
              tf.summary.scalar(tag, datum, step=step)
              step += 1
          writer.flush()
      self._multiplexer.AddRunsFromDirectory(self._logdir)
  
imports: import argparse
  import collections.abc
  import os.path
  import tensorflow.compat.v1
  import tensorflow.compat.v2
  from tensorboard import context
  from tensorboard.backend.event_processing import data_provider
  from tensorboard.backend.event_processing import plugin_event_multiplexer
  from tensorboard.data import provider
  from tensorboard.plugins import base_plugin
  from tensorboard.plugins.image import metadata
  from tensorboard.plugins.metrics import metrics_plugin
label_desc: Use image node_id 3 for calling the MetricsPluginTest obj's underlying member method code with example usage: obj._write_scalar_data(run, tag, data) without return types