Custom sorting in PowerBI chart

Add a column to your source data and populate it with values for the desired sort order. Then select the column you want to custom sort and click Modeling > Sort by Column. Select the new column as the SortBy column for the existing column, like in the screenshot: The SortCategory column is used as the sort order for the Category column.

Now create a chart and the categories on the axis will be sorted according to the values of the SortCategory column.