Controlling how plots order nominal values

Hi.

Suppose a user wants to create a "bars stacked" plot, with ordered stacking and grouping categories.
How can this be accomplished reliably? If the outputs from the following process are plotted, either the x-axis is out of order, or the stacking categories are out of order, depending on how the data is sorted. This is the minimal exampleset that induces this problem.

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<process version="5.3.005">
  <context>
    <input/>
    <output/>
    <macros/>
  </context>
  <operator activated="true" class="process" compatibility="5.3.005" expanded="true" name="Process">
    <process expanded="true">
      <operator activated="true" class="generate_data_user_specification" compatibility="5.3.005" expanded="true" height="60" name="Generate Data by User Specification" width="90" x="45" y="300">
        <list key="attribute_values">
          <parameter key="StackBy" value="&quot;red&quot;"/>
          <parameter key="GroupBy" value="&quot;b&quot;"/>
          <parameter key="Value" value="1"/>
        </list>
        <list key="set_additional_roles"/>
      </operator>
      <operator activated="true" class="generate_data_user_specification" compatibility="5.3.005" expanded="true" height="60" name="Generate Data by User Specification (2)" width="90" x="45" y="210">
        <list key="attribute_values">
          <parameter key="StackBy" value="&quot;blue&quot;"/>
          <parameter key="GroupBy" value="&quot;b&quot;"/>
          <parameter key="Value" value="1"/>
        </list>
        <list key="set_additional_roles"/>
      </operator>
      <operator activated="true" class="generate_data_user_specification" compatibility="5.3.005" expanded="true" height="60" name="Generate Data by User Specification (3)" width="90" x="45" y="390">
        <list key="attribute_values">
          <parameter key="StackBy" value="&quot;red&quot;"/>
          <parameter key="GroupBy" value="&quot;a&quot;"/>
          <parameter key="Value" value="1"/>
        </list>
        <list key="set_additional_roles"/>
      </operator>
      <operator activated="true" class="append" compatibility="5.3.005" expanded="true" height="112" name="Append" width="90" x="313" y="255"/>
      <operator activated="true" class="sort" compatibility="5.3.005" expanded="true" height="76" name="sort by GroupBy" width="90" x="514" y="255">
        <parameter key="attribute_name" value="GroupBy"/>
      </operator>
      <operator activated="true" class="sort" compatibility="5.3.005" expanded="true" height="76" name="sort by StackBy" width="90" x="648" y="255">
        <parameter key="attribute_name" value="StackBy"/>
      </operator>
      <connect from_op="Generate Data by User Specification" from_port="output" to_op="Append" to_port="example set 2"/>
      <connect from_op="Generate Data by User Specification (2)" from_port="output" to_op="Append" to_port="example set 1"/>
      <connect from_op="Generate Data by User Specification (3)" from_port="output" to_op="Append" to_port="example set 3"/>
      <connect from_op="Append" from_port="merged set" to_op="sort by GroupBy" to_port="example set input"/>
      <connect from_op="sort by GroupBy" from_port="example set output" to_op="sort by StackBy" to_port="example set input"/>
      <connect from_op="sort by StackBy" from_port="example set output" to_port="result 1"/>
      <connect from_op="sort by StackBy" from_port="original" to_port="result 2"/>
      <portSpacing port="source_input 1" spacing="0"/>
      <portSpacing port="sink_result 1" spacing="0"/>
      <portSpacing port="sink_result 2" spacing="0"/>
      <portSpacing port="sink_result 3" spacing="0"/>
    </process>
  </operator>
</process>

How can it be solved, so that both the stacking order and the x-axis order are correct?

(It is important to have consistently ordered colors/categories when multiple plots need to be compared within/across reports, for example.)

Regards
Isak