Can I predesign a chart in an Excel template and then get access to the predefined chart through the API? Or, when using Excel.Chart, can I only create a chart using the API?
Yes, you can predesign a chart in your designer spreadsheet(template). And you can modify the source data of the chart using Aspose.Excel.
But if my range of data changes for the predefined chart, can I then through Aspose.Excel, gain access to the chart to modify the charts range of values?
Now Aspose.Excel cannot change a range of data for a predefined chart. If you want to do it, you have to use APIs to create the chart.