Level 7 · Is the app catching up with the website?

Wide tables draw several lines

pandas draws one line per column when you call .plot() on a DataFrame, with the index on the x-axis and a legend built from the column names. So a chart with a line per channel needs a table with a column per channel. The starter gives you one row per order line with a month column (the first day of each month); pivot_table turns it wide:

wide = lines.pivot_table(index="month", columns="channel",
                         values="revenue", aggfunc="sum").round(2)
#                  app  marketplace        web
# 2025-01-01  38566.02     24323.19   87681.78
# 2025-02-01  46574.43      8919.22   44409.50
# ...
ax = wide.plot(marker="o")

.plot() returns the Axes, and from there it’s the usual matplotlib: ax.set_title, ax.set_ylabel, ax.set_ylim(0, None) to keep zero in view.

Doing it by hand

The same chart in plain matplotlib is one ax.plot per series, each with a label=, and a legend at the end:

fig, ax = plt.subplots()
for channel in ["app", "web", "marketplace"]:
    ax.plot(wide.index, wide[channel], marker="o", label=channel)
ax.legend()

ax.legend() only knows about lines that were given a label. Call it after the plotting, not before.

What the lines say

January started with the website far ahead: $87,681.78 against $38,566.02 for the app. From February the app led in every month except July and August, including the peaks of November ($118,601.68) and December ($130,471.52). Over the year the app brought $996,296.62 and the website $980,935.78, so Jordan’s answer is yes, narrowly. The marketplace stays a smaller, steady third line at $324,457.72 for the year.

Keep the number of lines small. Three lines are easy to follow; eight lines in eight colors are spaghetti. With many groups, draw small separate charts (next levels) or highlight one line and gray out the rest.

Common mistakes

• Plotting a long table with x="month": every channel’s points go into one zigzag line.
• Calling ax.legend() when no line has a label, which gives an empty legend.
• Letting the y-axis start at the lowest value, which exaggerates the gaps between lines.
• Leaving months as text like “2025-01”; real dates keep the axis in calendar order.
• Forgetting .round(2) or rounding before summing: round the totals, never the line values.