Choose chart category through a parameter

Hi, all.

I need to build a line chart where I can choose the category through a parameter.
Specifically in my case, I need to choose, through a dropdown, whether my line chart will display data by year or month.

I created a non-queried parameter, where the values are:
"=Fields!Year.Value"
"=Fields!Month.Value"

Then, I changed the chart's "Category Expression" from
=Fields!Month.Value
to
=Parameters!PrmPeriod.Value.

What I get is an empty line chart, with the label "=Fields!Year.Value" or "=Fields!Month.Value" (including the quotation marks).

I also tried to change the parameter values to just "Year" and "Month" and then I'd change the category Expression to
="Fields!" & Parameters!PrmPeriod.Value & ".Value"
but this wouldn't work either.

Can somebody help me with that or tell for sure whether this is possible?

Thx

Dan