SSRS line chart, 2 series, many data points, 1 category

Question

I am trying to create one SSRS report with SQL Server database at backend.
The chart is having Result Value on Y-axis and Result Date on X axis.

The result date needs to be shown like below, Day (1,2,3,4... etc) need not be continuous...

Sept, 2015 Oct, 2015 Nov, 2015
1 2 3 4 5 .... 1 2 3 4 5.... 1 2 3 4 5...

The report shows 2 series,
1. First test result value of Test point on a day. (single value per day)
2. Retest values for test point on a day (Multiple values per day)

Here's what I have done so far
1. inserted one table put up row grouping to test point name, so as to get a chart per Test Point
2. insert chart control in a cell, set up chart type as line chart.
3. Add 2 category groups. First group by Month (label as Monthname and Year) of Result Date and second group by Day (label as Day) of result date
4. Put up series value as First(Fields!ResultValue.Value) to get the first measurement for that day.

Everything looks okay till now, but now I am not able to plot the 2nd series i.e. retest values which are multiple values on same day

I have tried following
1. create series group on IsRetest field to have retest data as separate series but this returns only first value because of existing category group.
2. I tried creating separate dataset but when I try using LookUpSet method to return all retest result values for particular testpoint it doesn't render anything on the chart, although just LookUp method works well but again it return only the first value.

Any help or guidance in this query would be much appreciated. Thanks in advance.

Regards,
Sujay C