@RUKNUDDIN MOHAMMAD  There is no Out of the Box(OOTB) functionality available to include a Chart image in a custom section and export that section to PDF as a whole. The product does not support it.
 

When charts are rendered on browser, byte-code of the chart is copied from browser using a third party library canvg and later embedded into the PDF. This does not work when chart is included in a section and using HTMLtoPDF activity to export the whole section along with the chart because HTMLtoPDF uses pd4ml library.

You can try to include pyEmbedChartUsingPhantomJS HTML Fragment in your section. It seems this converts the image bytecode which will then be appended to PDF stream. Alternatively create data base view (it will solve multiple class reference issue) and  connect the view with report definition. Use the report definition export option.

The above local change is only applicable for charts sourced from Report Definition and is implemented as mentioned in the following link: 

https://community.pega.com/knowledgebase/articles/exporting-chart-scheduled-report-pdf-file

If its a JS based chart,  export to pdf of charts are not supported.  js based fusion chart wont render in pdf generation as there is no chart markup generated on server.

See:
https://community.pega.com/support/support-articles/issue-exporting-charts-pdf 

Note that Enhancement  FDBK-72382 exists for future consideration where  Pega Chart sourced from DT would  get exported to PDF.