# How to increase the thickness of the graph lines and remove the point?

It is necessary to remove the round and square dots on the graphs lines. And to increase the thickness of the line.

\begin{tikzpicture}
\begin{axis}[date coordinates in = x,
table/col sep = semicolon,
height = 0.3\paperheight,
width = 0.7\paperwidth,
xmin=2015-10-01,
xmax=2015-12-31,
ymin=30,
ymax=55,
/pgf/number format/1000 sep={}
]
\addplot table[x index=0, y index=1, col sep=comma] {oil_prices.csv};
\addplot table[x index=0, y index=2, col sep=comma] {oil_prices.csv};
\end{axis}
\end{tikzpicture}


    no markers,
every axis plot/.append style={ultra thick}


to the axis options. I don't have your data files, so in the example below I've used random numbers. Instead of ultra thick you can use any of the other default styles defined by TikZ (semithick, thick and very thick), or set the width explicitly using e.g. line width=4pt. (4pt is likely too much, but adjust to your liking).

Left is default, right is ultra thick without markers.

\documentclass{article}
\usepackage{pgfplots}
\pgfplotsset{width=5cm}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
name=ax1,
]
\end{axis}

\begin{axis}[
at={(ax1.outer north east)},anchor=outer north west,
no markers,
every axis plot/.append style={ultra thick}
]