How to crop figures in LaTex?

It is common when we inserting figures in the papers in LaTex where some of them can have a white border taking up a lot of space. The trim option in \includegraphics aims to crop the figures inside the LaTex environment, which makes manipulating figures easy. For example, 

\begin{figure}[!bh]
\centering
\includegraphics[width=.8\linewidth,
trim=0cm 0cm 0cm .4cm, clip=true]
{images/user_difference.pdf}
\caption{The relationship between the number of favorites and retweets for two different users in a log scale.}
\label{fig:user_diff}
\end{figure}

The four locations after trim= are [left bottom right top] respectively, and [0cm 0cm 0cm .4cm] means we are trimming top for .4cm of the figure.

We can also use the clip=true or clip=false to turn on and off the trimming of the figure.