Prompt parameter name as a prompt

Recently I was presented with an interesting problem. The developer made a chart report which allowed the user to define the x-axis through a radio button. In order to do this the developer dragged all the fields he needed into the query, and used the following expression to change the axis:

This means if Month was selected in the radio, the x-Axis would be [Month].

Next he set up a drill through to a list report. The user would click on one of the bars, and it would filter the detail report based on the category selected, and the bar clicked. The optional filter is:
#sb(prompt('Category','token'))# = #prompt('Value','string')#
Resolving to [Month] = ‘Jan/2010’ or maybe [Product] = ‘Glowsticks’

The target report is also designed as a standalone, with a dozen prompts on the page. And now we get to the problem. If the user drilled on Country = UK, and then selects France and Belgium, the report will return no results. This makes sense as the country filter is [Country] in (‘France’,’Belgium’) and the Category filter is [Country] = ‘UK’. The expected behaviour is slightly different. The user expects to override the category filter with whatever is selected in the regular prompts. It also has to be accomplished without JavaScript.

I’d welcome ideas on different ways to accomplish this, as the solution I’m about to present is difficult to understand and maintain.

First I changed all of the prompts on the page to match the values the categories were passing. So the filter [Country] = ?p_dest_country? was changed to [Country] = ?Country? and [Month] = ?p_Month? was changed to [Month]=?Month?

The categories filter was changed to:
#sb(prompt(‘Category’,’token’,’1′))# in (#promptmany(prompt(‘Category’,’token’),’string’,sq(prompt(‘Value’,’string’,’1′)))#)

Assuming the user drilled on Country = UK, this will resolve to:

[Country] in (#prompt(‘Country’,’string’,sq(‘UK’))#)

Since the Country parameter in the report is null, it will default to UK, ultimately resolving to:
[Country] in (‘UK’)

Since the drilled value is only in the default parameter of the macro, selecting any value from the prompt will always take precedence.

Please feel free to leave a comment if you can think of another way of handling this.