simple tfs burndown

I am trying to create a simple (I think) burndown report

I have the data from TFS which consists of all the bugs created since a specifc date

i just want to see the number of open bugs on a graph.

i create a column. "is still open", which determines whether the bug is still open (<> to closed or completed)

i also create a measure that sums the "is still open" colume

i created a line chart, where the Axis is the create date and Value is the Measure

the chart shows the constant value of 1, which means it is not showing the TOTAL number of bugs, but is showing each bug on the timeline as 1.

How can i represent the sum of the "is still open" as the value?