When i take a look at all the openmrs module-depencies for core apps module ie
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<emrapiVersion>1.26.0</emrapiVersion>
<uiframeworkVersion>3.15.0</uiframeworkVersion>
<eventVersion>2.5</eventVersion>
<htmlformentryVersion>3.3.0</htmlformentryVersion>
<appuiVersion>1.7</appuiVersion>
<appframeworkVersion>2.9</appframeworkVersion>
<openMRSVersion>1.12.0</openMRSVersion>
<serialization.xstreamVersion>0.2.12</serialization.xstreamVersion>
<calculationVersion>1.2</calculationVersion>
<reportingVersion>0.10.4</reportingVersion>
<providermanagementVersion>2.9.0</providermanagementVersion>
<idgenVersion>4.4.0</idgenVersion>
<metadatamappingVersion>1.3.2</metadatamappingVersion>
<uicommonsVersion>2.3.0</uicommonsVersion>
<webservicesRestVersion>2.17</webservicesRestVersion>
, and also looking at the general trend for most Apps in core apps , they have their Back-end API functionality in the appFrameworkService in the Appframework module.
I would suggest that theAppframework module and appFrameworkService would be the suitable Module and service respectively to contain the Backend Api functionality for the Growth charts.