Currently, it cannot be done from using the format/shading. However, as a workaround, a calculated field can be created to decide which columns are colored and a gantt chart can be used. I have reattached your workbook showing this. Number of Records is placed on the size shelf and then the size legend is adjusted so that they are all the same.
Hope this helps!
Column Shading.twbx.zip 328.4 KB
Thanks, did the trick.