code: def __draw_reference_rule(ref_group, chart_height, chart_width):
      """Draws vertical reference rule where the ratio is the same as the reference group."""
  
      reference_rule = (
          alt.Chart(DUMMY_DF)
          .mark_rule(
              strokeWidth=Reference_Rule.stroke_width,
              stroke=Reference_Rule.stroke,
              strokeDash=Reference_Rule.stroke_dash,
          )
          .encode(
              x=alt.value(chart_width / 2),
              y=alt.value(
                  chart_height * Disparity_Chart.padding_y / 1.2
              ),
              y2=alt.value(
                  chart_height * (1 - Disparity_Chart.padding_y / 1.2)
              ),
              tooltip=alt.value(f"{ref_group} [REF]"),
          )
      )
  
      return reference_rule
  
imports: import math
  import altair
  import pandas
  from aequitas.plot.commons.helpers import no_axis, transform_ratio, get_chart_metadata
  from aequitas.plot.commons.legend import draw_legend
  from aequitas.plot.commons.scales import get_chart_size_range
  from aequitas.plot.commons.tooltips import get_tooltip_text_group_size, get_tooltip_text_disparity_explanation
  from aequitas.plot.commons.style.classes import Metric_Axis, Axis, Annotation, Reference_Rule, Threshold_Rule, Threshold_Band, Bubble, Chart_Title
  from aequitas.plot.commons.style.text import FONT, FONT_SIZE_SMALL
  from aequitas.plot.commons.style.sizes import Disparity_Chart
  from aequitas.plot.commons import initializers
  from aequitas.plot.commons import labels
label_desc: Use image node_id 6 for calling a global function with example usage: __draw_reference_rule(ref_group, chart_height, chart_width) and returns: reference_rule