I created a list of 2,500 items and the chart displays the correct data for all 2,500 records.
I did the same for qSIChartView and same result.
I was using QWP 5.5 and SP2010.
If you could provide your list template (including the data) I'd be happy to test in my env.
Also please check if you have any SharePoint configuration that would limit your list size.