Issues Filtering a chart

I am trying to filter a series in a chart by specifying
a paramter. I am showing a chart where the data
returned from the SP has many employees, but I only
want the employee specified in the parameter. However,
when I enter the following:

=Parameters!employee_id.Value

into the filters tab for the employee series, the chart
is blank and I know for sure that there is data for the
employee I have specified in the parameter because
when I remove the filter, that employee shows up in
the legend along with 20 others.

This should be simple and it's driving me nuts!
Can someone point out what I'm doing wrong?

Turns out the SSRS "like" operator does not
ignore spaces like SQL does. I had spaces
coming back at the end of each employee so
I had to modify the above as follows:
=Parameters!employee_id.Value + "*"