Creating Trend line (scorecard) from data rather than counts

I need to create a KPI similar to the  DeepSee.Model.KPIs.DemoTrendLines kpi in Samples.  This kpi supports a scorecard widget with a trend line.  It shows patient counts by city with a trend line of the count of patients over a decade.  What I want is to show the trend line based on the average allergy score ([Measures].[Avg Test Score]).  A pivot table query that shows the data I would want to base the trend line on is:

SELECT NON EMPTY [BirthD].[H1].[Decade].Members ON 0,
NON EMPTY [HomeD].[H1].[City].Members ON 1 
FROM [Patients] %FILTER [Measures].[Avg Test Score]

Has anyone done this?