diff --git a/doc/Si-kdeplot.png b/doc/Si-kdeplot.png
index 4138a4ed..8d4ebdd1 100644
Binary files a/doc/Si-kdeplot.png and b/doc/Si-kdeplot.png differ
diff --git a/doc/auxiliary-tools.rst b/doc/auxiliary-tools.rst
index dfaa28d0..6521c2a7 100644
--- a/doc/auxiliary-tools.rst
+++ b/doc/auxiliary-tools.rst
@@ -214,7 +214,8 @@ the first argument::
 
    % kdeplot --nbins=200 kappa-m191919.hdf5
 
-After finishing calculation, the plot is saved in
+(This may take long time. On a relatively fast machine, it took 20 mins.)
+After finishing the calculation, the plot is saved in
 ``lifetime.png``. The black dots show the phonon modes. The density is
 estimated from these dots. The drawing area is automatically set to
 make look good, e.g, higher lifetime side is not drawn if the density