Thanks for the screenshot and details.
I think it is MS Excel's behavior which does not wrap text of category axis when you set the angle to 45 degree. So, there is nothing to do with Aspose.Cells APIs. You got to specify the angle to 90 degree or horizontally, if you want the text should be wrapped. Aspose.Cells follows MS Excel standards and specifications. If you think it is possible in MS Excel, kindly do create the chart and specify your category axis labels wrapped @ 45 degree angle manually in MS Excel and save the file to provide us here, we will check it soon.