I want to create a map of India showing 30 districts that have the highest COVID-19 cases as of 31 December 2020 using QGIS. I marked the 30 districts and added the CSV file to this layer. I want to create a proportional symbol according to cumulative confirmed cases, but not with discrete values, but for specific ranges because the number of cumulative cases ranges from 55000 to 6 lakhs. I searched for the "Assistant' option in the 'size' section in the 'Properties' of the point. But in the input menu I am not able to give the source. It doesn't show anything for selection.So if I apply my changes, it is displaying graduated symbols not proportional symbol.
How can I solve this? Why does the input menu display nothing? I want to gave the column name of the confirmed cases in the csv file in that section. But it shows nothing.