Pie chart with prometheus

I want to use the pie chart to show how many times certain APIs are called during the selected time period. My data source is prometheus so I use increase(count[15s]) with a min step 15s, and “total” option to calculate the data. That way can get the correct result in the build-in graph panel, but the result is always quite weird in the pie chart: the “total” option never returns the right result. Is the “total” option in pie chart has a different mean from the graph panel? If so, what should I do to get the result of the sum of the data points during the selected time period?