Hi VM,
Thanks for finding this! After researching I found the missing XML schema and it will be added to an upcoming release of MS-ODRAWXML. In the meantime, below is the schema, so you can move forward with your project.
<xsd:schema targetNamespace="http://http://schemas.microsoft.com/office/drawing/2015/06/chart"
elementFormDefault="qualified"
attributeFormDefault="qualified"
xmlns="http://schemas.microsoft.com/office/drawing/2015/06/chart"
xmlns:c="http://schemas.openxmlformats.org/drawingml/2006/chart"
xmlns:xsd="http://www.w3.org/2001/XMLSchema"
blockDefault="#all">
<xsd:import namespace="http://schemas.openxmlformats.org/drawingml/2006/chart" schemaLocation="chart.xsd" />
<xsd:attribute name="formatcode2" type="c:ST_Xstring" />
<xsd:element name="formatcode2" type="c:ST_Xstring" />
</xsd:schema>
Best regards,
Michael Bowen
Microsoft Open Specifications