Unfortunately, I don't know fortesreport. So let's go step by step:
When you have your chart on a standard panel, outside the report, or in a standard form, is the chart showing fine? If yes, the issue is not your chart drawing code.
Next, you should look at the sample demo in (lazarus)\components\tachart\demo\lazreport. This is a demo for the integration of TAChart into lazreport. The main idea is that you repaint the chart on a bitmap which you show in an image component of the report. This is pretty universal and should work also with fortesreport.