Uniaxial compression test data for a solid metal bar of length $1$ m is shown in the figure.

The bar material has a linear elastic response from $O$ to $P$ followed by a nonlinear response. The point $P$ represents the yield point of the material. The rod is pinned at both the ends. The minimum diameter of the bar so that it does not buckle under axial loading before reaching the yield point is _______ mm ( round off to one decimal place)