Chart Widget

I am using OTB chart widget and would like to be able to summarize the different types of pipe material in the pie chart. For example ac, ci, di, pvc. This is what I have so far:
<configuration>
<labels>
<layerselectionlabel>Select layer</layerselectionlabel>
<drawtoolmenulabel>Select draw tool</drawtoolmenulabel>
</labels>
<layers>
<layer label="Water Pipe Material" url="http://wr_modeling/ArcGIS/rest/services/UtilitiesDashboard/MapServer/12">
<labelfield>MATERIAL</labelfield>
<fields>
<field name="MATERIAL"/>
<field name="SHAPE_LEN"/>
</fields>
<medias>
<media chartfields="SHAPE_LEN"
title="Length of Pipe Material"
type="piechart"/>
</medias>
</layer>

</layers>
</configuration>