# filling the area under a plotted function in tikz

For the shown MWE:

• I need to fill the area under the function plotted in tikz using the "Plot Command". However, using the "Fill" option in the "Draw command" does not yeild any results.

• I need to fill the area under the plot in one occasion and in another I need to fill the area above the plot.

• Also can I replace the fill with a pattern such as vertical lines....

• In adition, is there a way to fill/hatch only a certain region within such plot?

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz}
\begin{document}
\begin{center}
\begin{tikzpicture}
\draw[help lines,step=0.5](0,0) grid(15,24);
\begin{scope}[shift={(0,6)}]
\draw[thick,color=red,fill=yellow,domain=0:6] plot (\x, {0.1*(5*-1)*(\x*6)/(12)});
\draw[thick,color=red,fill=yellow,domain=6:12] plot (\x, {0.1*(5*-1)*((12-\x)*(6))/(12)});
\end{scope}
\end{tikzpicture}
\end{center}
\end{document}


• If you want to draw graphs, look up data visualization (TikZ) or use pgfplots (and fill between). Otherwise, provide a complete path for \fill, including all sides. – John Kormylo Nov 20 '20 at 14:31
• @ John Kormylo, whey use "Data visualization" while plotting a function using a math engine is most accurate? – Silva Nov 20 '20 at 14:35
• Datat visualization has more to do with axes, grids and converting "furlongs per fortnight" into cm. Also, pgfmath is not accurate by any stretch of the immagination. – John Kormylo Nov 20 '20 at 14:39
• However, mkaing a path or bezier curve for parabolic curves (3rd, 4th degree equations and above) is troblesome, that is why using plot equations is easier in such casese. I only used this function as a simple example however, there are occasions where I need to plot 4th degree equations, making a path or bezier curve for such equation, or even using the Hobby package is very troublesome in such case. – Silva Nov 20 '20 at 16:00
• What does "the area under the plot" mean? -Vertically down to the bounding box? What do you mean by "a certain region"? – hpekristiansen Nov 20 '20 at 19:43

Code:

\documentclass[tikz,border=12mm]{standalone}

\begin{document}

\begin{tikzpicture}
\draw[help lines,step=0.5](0,0) grid(15,10);
\begin{scope}[shift={(0,6)}]
\fill [yellow] (0,-1.5) -- plot[domain=0:6]  (\x, {0.1*(5*-1)*(\x*6)/(12)});
\draw[thick,color=red,domain=0:6] plot (\x, {0.1*(5*-1)*(\x*6)/(12)});

\fill [yellow] (6,0)-- plot[domain=6:12]  (\x, {0.1*(5*-1)*((12-\x)*(6))/(12)});
\draw[thick,color=red,domain=6:12] plot (\x, {0.1*(5*-1)*((12-\x)*(6))/(12)});
\end{scope}
\end{tikzpicture}

\end{document}


output:

code2:

\begin{tikzpicture}
\draw[help lines,step=0.5](0,0) grid(15,10);
\begin{scope}[shift={(0,6)}]
\fill [yellow]  plot[domain=0:6]  (\x, {0.1*(5*-1)*(\x*6)/(12)})--(6,0);
\draw[thick,color=red,domain=0:6] plot (\x, {0.1*(5*-1)*(\x*6)/(12)});

\fill [yellow] (6,0)-- plot[domain=6:12]  (\x, {0.1*(5*-1)*((12-\x)*(6))/(12)});
\draw[thick,color=red,domain=6:12] plot (\x, {0.1*(5*-1)*((12-\x)*(6))/(12)});
\end{scope}
\end{tikzpicture}


output:

code3:

\begin{tikzpicture}
\draw[help lines,step=0.5](0,0) grid(15,10);
\begin{scope}[shift={(0,6)}]
\fill [yellow] (0,-1.5) -- plot[domain=0:6]  (\x, {0.1*(5*-1)*(\x*6)/(12)});
\draw[thick,color=red,domain=0:6] plot (\x, {0.1*(5*-1)*(\x*6)/(12)});

\fill [yellow]  plot[domain=6:12]  (\x, {0.1*(5*-1)*((12-\x)*(6))/(12)})--(12,-1.5);
\draw[thick,color=red,domain=6:12] plot (\x, {0.1*(5*-1)*((12-\x)*(6))/(12)});
\end{scope}
\end{tikzpicture}


output: