A chart that goes into a report needs a title and labeled axes. You can set them in the plot call:
monthly.plot(title="Lakeside revenue by month", xlabel="Month", ylabel="Revenue ($)")
plot returns a matplotlib Axes, the area the chart is drawn in. Keep it to change things afterwards:
ax = monthly.plot()
ax.set_title("Lakeside revenue by month")
ax.set_ylabel("Revenue ($)")
figsize=(8, 4) sets the width and height in inches. To save the chart as an image for a slide, get its figure and save it: ax.get_figure().savefig("revenue.png", bbox_inches="tight").