This is what I've written so far, but I'm not sure how to add the values at the top of each bar.
# Visualize monthly revenues for top 4 sources.
top_four.pivot(index = 'month', columns = 'source_id', values = 'revenue').plot(kind='bar',
figsize=(20, 10),
color=['cornflowerblue','salmon','powderblue','darkseagreen'],
rot=0)
plt.title("Monthly Revenue for Top 4 Source ID's", y = 1.03, fontsize=30)
plt.xlabel("Date [Month]", labelpad=24, fontsize=20)
plt.ylabel("Revenue in USD", labelpad=20, fontsize=20)