diff --git a/slides/sections/4.md b/slides/sections/4.md
index f3a1c97..384a869 100644
--- a/slides/sections/4.md
+++ b/slides/sections/4.md
@@ -32,8 +32,8 @@ How to estimate sample median, mode and FWHM?
 \setbeamercovered{}
 \begin{center}
 \begin{tikzpicture}[remember picture, >=Stealth]
-  % place holder
-  \draw [ultra thick, white] (-0.35,0.7) -- (-0.35,-0.7);
+  % placeholder
+  \draw [ultra thick, transparent] (-0.35,0.7) -- (-0.35,-0.7);
   % line
   \draw <1-> [line width=3, ->, cyclamen] (-5,0) -- (5,0);
   \node <1-> [right] at (5,0) {$x$};
@@ -139,7 +139,7 @@ $$
   \begin{center}
   \begin{tikzpicture}
     % placeholder
-    \draw [white] (-2.7,-0.2) rectangle (3,3.3);
+    \draw [transparent] (-2.7,-0.2) rectangle (3,3.3);
     % bandwidth 1
     \node <4,5> [left] at (2.9,3) {$\epsilon = 1$};
     % points
@@ -162,7 +162,6 @@ $$
     \draw <4,5> [domain=-0.1:2.9, smooth, variable=\x, cyclamen, very thick]
                plot ({\x}, {exp(-(\x - 1.4)*(\x - 1.4)) + 0.1});
     % sum 1
-    \draw <5> [fill=white, white, opacity=0.5] (-2.7,0.1) rectangle (3,2.7);
     \draw <5> [domain=-2.6:2.9, smooth, variable=\x, yellow, very thick]
                 plot ({\x}, {exp(-(\x + 1.1)*(\x + 1.1)) +
                              exp(-\x*\x) +
@@ -185,7 +184,6 @@ $$
     \draw <6> [domain=-0.1:2.9, smooth, variable=\x, cyclamen, very thick]
                plot ({\x}, {exp(-(\x - 1.4)*(\x - 1.4)/0.25)/0.5 + 0.1});
     % sum
-    \draw <6> [fill=white, white, opacity=0.5] (-2.7,0.05) rectangle (3,2.7);
     \draw <6> [domain=-2.6:2.9, smooth, variable=\x, yellow, very thick]
                 plot ({\x}, {exp(-(\x + 1.1)*(\x + 1.1)/0.25)/0.5 +
                              exp(-\x*\x/0.25)/0.5 +