oox export of chart transparency

Hi all,

in connection with discussions in bug 108065, I looked at the export of 
transparency in charts.
https://bugs.documentfoundation.org/show_bug.cgi?id=108065
[FILESAVE DOCX] Gradient property of the data series not preserved when 
saved as DOCX

The problem has several reasons
a) void DrawingML::WriteGradientFill(awt::Gradient rGradient,
     const uno::Reference<beans::XPropertySet>& rXPropSet)
expects a property "FillTransparenceGradient" but in charts only 
"FillTransparenceGradientName" is available.
b) void ChartExport::exportFill( const Reference< XPropertySet >& xPropSet )
does not consider transparency gradient in case of solid fill.
c) import of a chart considers transparency only on data point filling.

What I have done so far is in https://gerrit.libreoffice.org/#/c/81165/1

My questions:
To get a gradient form "FillTransparenceGradientName", I have found 
"com.sun.star.drawing.TransparencyGradientTable". To use it, I need a 
model. That is available in class ChartExport, but not in the base class 
DrawingML. Is there a way to get the gradient itself from its name in 
DrawingML::WriteGradientFill()?

My idea for the export works for charts in Calc and Writer. But any 
(e.g. a default shape without any gradient or transparency) export from 
odp to pptx crashes. So I'm doing something fundamental wrong. But what?

Kind regards
Regina