I am having trouble with some custom code, I want to get the range of my data and add a line midway

I tested out some code that works ok for a floatbar in the winforms charting object, and tried to use it in the reporting chart tool custom code.

double test = (double)floatBar.Values[0];

As soon as i put this code into my report it no longer runs as says there is an error, as soon as i remove  it it runs again.  Same for BeginValues, EndValues etc.

Am i doing something wrong?  I have two level grouping on my report but i'm not sure if that makes a difference.