Inserting boundary box around graph

Question

I wish to insert a black boundary box around a graph. I have searched through the forum but I couldn't find a way to do so. The image attached is how my graph looks like.

Below is my document code:

\documentclass[11pt]{article}
\usepackage{graphicx}
\usepackage{float}
\usepackage{grffile}
\begin{document}
\begin{figure}[H]
\centering
\includegraphics[height = 3in]{/Users/kusha/Desktop/graphs/kden_2011.pdf}
\caption{Kernel density curves of male and female wages in 2011}
\label{fig:kden_2011}
\end{figure}
\end{document}