At the advanced statistics screen different chart types are supported. The to be used chart type can be set at question level at the advanced question settings.
When selecting PDF as output, only bar and pie charts are used and the question based setting is completely ignored.

I know that for technical reasons a different charting library is used for generatng the PDF. But even the old pchart supports line and radar graphs so maybe that can be implemented as well?

As a short term solution I recommend adding a note at the statistics screen saying:
"For technical reasons the Excel output does NOT contain any charts. At PDF output charts are limited to pie charts (single choice questions) and bar charts (multiple choice question types)."