Thanks for the feedback!
I've anonymized and am attaching the field chooser portion of my report and the first search panel.
I've tried each of cmerriman's suggestions without success.
Regarding nfillippi's suggestion, I've confirmed that the search handler is resolving all dependencies by selecting "Open in Search" from below the Dashboard panel, and confirming that the query expands properly. I've also tried a suggestion I've seen on the web stating that double "$" signs should be used around the field variable, but this doesn't make a difference.
Any further input appreciated!
Reports
<input type="dropdown" token="series" searchWhenChanged="true">
<label>Pick an XXX by Name</label>
<choice value="*">All</choice>
<default>*</default>
<search>
<query>index=xxx sourcetype=XXX_HEADER_TYPE | stats count by XXX_NAME</query>
</search>
<fieldForLabel>XXX_NAME</fieldForLabel>
<fieldForValue>XXX_NAME</fieldForValue>
</input>
<panel>
<chart>
<title>Distribution of XXX Categories Real Time last 60 min</title>
<search>
<query>search autorun="true" index=xxx sourcetype=XXX_HEADER_TYPE | search XXX_NAME=$series$ | eval CATEGORY=case(U_XXX >4, "5:Excellent", U_XXX > 3, "4:Good", U_XXX >=2, "3:Fair", U_XXX >=1, "2:Poor", U_XXX <11, "1:Bad") | timechart count(CATEGORY) by CATEGORY | fields _time, 5:Excellent, 4:Good, 3:Fair, 2:Poor, 1:Bad</query>
<earliest>rt-1h</earliest>
<latest>rt</latest>
</search>
<option name="charting.axisTitleX.visibility">visible</option>
<option name="charting.axisTitleY.visibility">visible</option>
<option name="charting.axisX.scale">linear</option>
<option name="charting.axisY.scale">linear</option>
<option name="charting.chart">column</option>
<option name="charting.chart.nullValueMode">gaps</option>
<option name="charting.chart.sliceCollapsingThreshold">0.01</option>
<option name="charting.chart.stackMode">stacked100</option>
<option name="charting.chart.style">shiny</option>
<option name="charting.drilldown">all</option>
<option name="charting.layout.splitSeries">0</option>
<option name="charting.legend.labelStyle.overflowMode">ellipsisMiddle</option>
<option name="charting.legend.placement">right</option>
<option name="wrap">true</option>
<option name="rowNumbers">false</option>
<option name="dataOverlayMode">none</option>
<option name="drilldown">cell</option>
</chart>
</panel>
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