SSRS 2005 line chart based on group avg question

Hi,  I appear to be unable to sort out what appears to be a fairly simple thing i'm trying to do with a line chart.

I have a simple table with two groups.  Group 1 is machine and Group 2 is date.  The details contain multiple weight measurements taken on each date.  Group 2 contains a group footer that displays the average weight for that day.

What i would like to do to is add a line chart into the Group 1 footer that displays this average weight per group 2 (in other words display a line chart of average weight per day per machine).  I've tried a multitude of things with scopes, aggregates etc but can't get this to work - I either get an error regarding scope parameters that aren't valid for an aggregate function or a line chart with a single point in it.  For example, setting the value field to =avg(fields!averageWeight.value, "group1") gives me a single point on the line whereas =avg(fields!averageWeight.value, "group2") gives me the error.

Please could someone advise what i may need to do to get this working - I know the answer is likely to be ridiculously simple but I've wasted too much time on this already!!!???

Thanks in advance