<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def plot_legend(ax, no_legend=True, legend_arg=None):
 """ Function that defines the legend options of a matplotlib plot. Args: ax: matplotlib axes no_legend (bool):
 Defines the presence of a legend in the figure legend_arg (dict):
 Addition arguments for matplotlib.legend() call """

    legend_arg = dict_if_none(legend_arg)

    if not no_legend:
        ax.legend(**legend_arg)