\documentclass[border=0mm]{standalone}

\usepackage{cmap}
\usepackage[cp1250]{inputenc}	
\usepackage[T1]{fontenc}
\usepackage{graphicx}
\usepackage{lmodern}
\usepackage{tikz}
\usepackage{float}
\usepackage{siunitx}
\usepackage{amsmath}
\usepackage{upgreek}
\usepackage{pgfplots}
\pgfplotsset{compat=1.17}
\usepgfplotslibrary{fillbetween}
\usepgfplotslibrary{units}
\usepgfplotslibrary{statistics}
% \makeatletter
%\pgfplotsset{
%  unit code/.code 2 args=
%    \begingroup
%    \protected@edef\x{\endgroup\si{#2}}\x
%} 
%\makeatother

\pgfkeys{%
  /tikz/on layer/.code={
    \pgfonlayer{#1}\begingroup
    \aftergroup\endpgfonlayer
    \aftergroup\endgroup
  }
}

\begin{document}
	\begin{tikzpicture}
		\begin{axis}[
    height=7cm,	width=11cm,
    x unit=\SI{}{\pico\second}, y unit=-,
	xlabel=Jitter,	ylabel=Normalized frequency,
    ybar,
    ymin = 0,
    %ymax = 50,
    xmin = -8, 
    xmax =8,
    xtick distance = 2,
    xtick pos=left,
	ytick pos=left
]

\addplot +[
    hist={density,
        bins=16,
        %data min=0,
        %data max=32
    }   
] table [y index=1, y expr=\thisrow{y}*1e12, col sep=semicolon] {../cadence_data/time_jitter_PLL_16.csv};
\addlegendentry{unlocked ILRO};

\addplot +[
    hist={density,
        bins=16,
        %data min=0,
        %data max=32
    },opacity=0.75   
] table [y index=1, y expr=\thisrow{y}*1e12, col sep=semicolon] {../cadence_data/time_jitter_ILO_16.csv};
\addlegendentry{locked ILRO};
\end{axis}
	\end{tikzpicture}
\end{document}