5 sigma contours

I am trying to draw 5 sigma contours in a two dimentional histo,
to show the discovery contour for a specific physics channel.
Is there any tool that I can use easily with dragging the mouse and
plot a curve of arbitrary shape like 5 sigma curves that are seen in discovery or exclusion limit plots?
I am trying to plot something like what I have attached.

You can use TGraph(s) to do that.