# Tikz-Draw smooth path to form a spectrum peak

I am using tikz to recreate the following image

What I am trying to do is draw an "extended" and "smooth" trapezoidal shape. My code is

\documentclass{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
%axes
\draw[very thick] (0,0)--(8,0);
\draw[very thick] (0,0)--(0,5) coordinate (y);
\draw (8,0) node[below] {\eng{Energy}};
\node [draw=none,rotate=90, yshift=0.3cm] at (0,5) {\eng{Counts}};
%1st peak
\draw[thick,blue] (7.3,0)--(7,3)--(5,4)--(4.7,0);
\draw[blue,dashed] (7,0)--(7,3);
\draw[blue,dashed] (5,4)--(5,0);
\draw[blue] (7,3) node[above,right] {$4$};
\draw[blue] (5,4) node[above,left] {$3$};
\draw[blue] (6,4.5) node[above] {$\delta E$};
%2nd peak
\draw[thick,red] (3.3,0) -- (3,2.5) -- (1,3.5) -- (0.7,0);
\draw[red,dashed] (3,0)--(3,2.5);
\draw[red,dashed] (1,0)--(1,3.5);
\draw[red] (3,2.5) node[above,right] {$1$};
\draw[red] (1,3.5) node[above,left] {$2$};
\draw[red] (2,4.5) node[above] {$\delta E$};
\end{tikzpicture}
\end{document}


and my output is

What I'd like to do is make the angles 1,2,3,4 smoother. Also I'd like to make a bit smoother-like having a small tail-the red and blue lines that touch the x-axis. How can these be achieved?

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I'm a bit confused. Isn't the shape you request built from two seperate normal (Gaussian) curves? Why not connect the average of that with two controls key'd curves? It seems to me like that makes more sense. – 1010011010 Aug 20 '14 at 9:46
@1010011010 : Thank's for your comment! It's not necessarily a Gaussian curve. Actually it's not a Gaussian curve. The dashed lines in the first image, are perpendicular to the x axis. – Thanos Aug 20 '14 at 9:49
Maybe my eyes are going bad, but the dashed lines in the first image don't look perpendicular to the x-axis to me. :-) – Paul Gessler Aug 20 '14 at 11:48
@PaulGessler : Your eyes are working perfectly fine! They're supposed to be, but they're not! It's just a bad image! – Thanos Aug 20 '14 at 11:56

I'm not quite sure what you mean with "extend the two perpendicular to the x-axis lines" but at least drawing a smooth line can be achieved with Bezier curves.

On a straight line like (a) -- (b) you can add (multiple) control point(s) x to create a smooth curve with (a) .. controls (x) .. (b)

(Taken from here)

EDIT: Thanks for editing your question :) let me then revise my answer...

Actually there is a "smooth" modifier for plotting. Additionally, I added another coordinate at the end and at the beginning and increased the y-value of the neighboring coordinates:

\draw[thick, green] plot [smooth] coordinates {(3.5,0) (3.3,0.5)  (3,2.5)  (1,3.5) (0.7,0.5) (0.5,0)};


For comparison the answer of Harish Kumar (both overlaid in green), also with two additional nodes to achieve smoothing along the x-axis.

\draw[thick,green, rounded corners=4mm ] (7.7,0) -- (7.3,0)--(7,3)--(5,4)--(4.7,0) -- (4.3,0);


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Thank's for your answer! It's not easy however to control point 2. \draw[thick,red] (3,0) .. controls (3,2.5) and (1,3.5) .. (1,0); imgur.com/4vOfKZe – Thanos Aug 20 '14 at 9:54
@Thanos Yes, you probably need more points for that: \draw[thick,red] (3,0) .. controls (3,2.5) .. (2,3.0) .. controls (1,3.5) .. (1,0); – Harald Aug 20 '14 at 10:03
I've edited my question! Thank you very much for your help and time! – Thanos Aug 20 '14 at 10:16

You can use rounded corners:

\documentclass{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
%axes
\draw[very thick] (0,0)--(8,0);
\draw[very thick] (0,0)--(0,5) coordinate (y);
\draw (8,0) node[below] {Energy};
\node [draw=none,rotate=90, yshift=0.3cm] at (0,5) {Counts};
%1st peak
\draw[thick,blue,rounded corners=4mm] (7.3,0)--(7,3)--(5,4)--(4.7,0);
\draw[blue,dashed] (7,0)-- node[pos=1.1,above,right] {$4$}(7,2.6);
\draw[blue,dashed] (5.1,0)-- node[pos=1.1,above,left] {$3$}(5.1,3.7);
\draw[blue] (6,4.5) node[above] {$δE$};
%2nd peak
\draw[thick,red,rounded corners=4mm] (3.3,0) -- (3,2.5) -- (1,3.5) -- (0.7,0);
\draw[red,dashed] (2.9,0)-- node[pos=1.1,above,right] {$1$}(2.9,2.3);
\draw[red,dashed] (1.1,0)-- node[pos=1.1,above,left] {$2$}(1.1,3.25);
\draw[red] (2,4.5) node[above] {$δE$};
\end{tikzpicture}
\end{document}


I have also made some simplifications in the code for nodes 1, 2, 3 and 4.

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Thank you very much for you answer! It was a great help! I've picked Harald's answer for two reasons. Mainly because it consists of two answers(his and yours) and therefore it's more complete and secondly because he has less reputation :P Just for the record, I am using your suggested rounded corners option! – Thanos Aug 20 '14 at 12:19
@Thanos Thank you! It even consists of three answers (the controls one, too) :P – Harald Aug 20 '14 at 12:47
@Harald : Indeed it does! :P Thank's again! – Thanos Aug 20 '14 at 12:50
@Thanos Oh No! ;-) Joking, it is OK. You can always upvote my other answers (as a compensation :P) if you like them. – Harish Kumar Aug 20 '14 at 23:07
@HarishKumar I certainly did so after "borrowing" your answer :) – Harald Aug 21 '14 at 8:46