Thanks for inquiring Aspose.Slides.
I have investigated the requirements shared by you and like to share that I have not been able to observe any option in PowerPoint even to change the value axis values to custom text. One can only change the values to predefined formats Can you please share the feedback about how to set the value axis to custom text.
The only work around that I can offer you in this regard is that you can generate the chart using Aspose.Slides. Hide the actual value axis and then add the custom text fields to serve the purpose. The following code snippet is used for creating chart and hiding the value axis.
PresentationEx pres = new PresentationEx();
SlideEx slide = pres.Slides;
ChartEx chart = slide.Shapes.AddChart(ChartTypeEx.ClusteredColumn, 150, 50, 500, 400);
chart.ValueAxis.IsVisible = false;
pres.Write(path + “Tempchart.pptx”);