If you attach a sample notebook it will be easier to answer your question.
1) Drag the calculated field which you created to detail marks section
2) change the mark section to pie
3) Drag the city dimension to angle
4) Once drag the dimension to angle --> Right click on City dimension --> measure --> countD
5) Drag the calculated field to Color
then you can see pie chart with count of city per calculated field with angel
Hope this helps kindly mark this answer as correct or helpful so that it will help others
this not that simple. The calculated field is a a discreate measure so when I am putting it in color only one category is showing . pie chart is not forming.
If its the case could you please attach a sample workbook