SSRS Use a formula for a line graph y = mX + b

Hi I want to show a line on a line chart using a formula in the form y = mX + b.   Can't seem to get this to work.   I inputted the formula into the "Value field" of the "Data field" but the line doesn't show up.

X = value for X axis (it's a date);  m and b are query fields (both type real)    The data is grouped on two fields that are specified in the "series field"

There are multiple values for X but only one value for M and B.

So for all dates (X) for each Group 1 AND Group 2 combo: chart this line.

Using: SSRS Visual Studio 2008