Waterfall chart pattern in Matplotlib - Step-by-Step Execution

Concept Flow - Waterfall chart pattern

Start with initial value

↓

Add positive changes

↓

Subtract negative changes

↓

Calculate intermediate totals

↓

Plot bars step-by-step

↓

Show final total bar

↓

End

A waterfall chart starts with an initial value, then adds or subtracts changes step-by-step, showing intermediate totals and a final total bar.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt
values = [100, -20, 30, -10, 40]
labels = ['Start', 'Loss', 'Gain', 'Loss', 'Gain']
plt.bar(range(len(values)), values)
plt.show()

This code plots simple bars for each value, but does not yet show the waterfall pattern with cumulative totals.

Execution Table

Step	Index	Value	Running Total	Bar Start	Bar Height	Bar Color
1	0	100	100	0	100	blue
2	1	-20	80	80	20	red
3	2	30	110	80	30	blue
4	3	-10	100	110	10	red
5	4	40	140	100	40	blue
6	Total		140	0	140	green

💡 All values processed, final total bar shows cumulative sum 140

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
running_total	0	100	80	110	100	140	140
bar_start	0	0	80	80	110	100	0
bar_height		100	20	30	10	40	140
bar_color		blue	red	blue	red	blue	green

Key Moments - 3 Insights

Why does the bar start position change for negative values?

Negative values are drawn downward from the current running total, so the bar start is the running total before this step (see execution_table rows 2 and 4).

How is the final total bar different from other bars?

The final total bar starts at zero and its height is the cumulative sum of all values, shown in execution_table row 6 with color green.

Why do some bars have red color and others blue?

Positive values use blue bars to show increases, negative values use red bars to show decreases, as tracked in variable_tracker 'bar_color'.

Visual Quiz - 3 Questions

Look at the execution table, what is the running total after step 3?

A 80

B 110

C 100

D 140

💡 Hint

Check the 'Running Total' column at step 3 in the execution_table.

At which step does the bar color change to red for the first time?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the 'Bar Color' column in execution_table rows.

If the value at index 1 changed from -20 to -50, what would be the new running total after step 2?

A 50

B 80

C 30

D 100

💡 Hint

Subtract 50 from the initial 100 to find the new running total after step 2.

Concept Snapshot

Waterfall chart pattern:
- Start with initial value
- Add/subtract changes stepwise
- Bars start at previous total
- Positive bars in one color, negative in another
- Final bar shows total sum

Full Transcript

A waterfall chart visually shows how an initial value changes step-by-step by adding or subtracting amounts. Each bar starts where the previous bar ended, with positive changes going up and negative changes going down. The final bar shows the total after all changes. This pattern helps understand how individual parts affect the whole.

Practice

1. What is the main purpose of a waterfall chart in data visualization?

easy

A. To plot continuous data over time

B. To display the distribution of data points in categories

C. To show step-by-step changes from a starting value to an ending value

D. To compare multiple variables side by side

Solution

1. Step 1: Understand waterfall chart concept

   A waterfall chart visually breaks down how an initial value changes through a series of positive and negative steps to reach a final value.

2. Step 2: Compare with other chart types

   Other charts like histograms or line charts serve different purposes, such as distribution or trends over time, not stepwise changes.

3. Final Answer:

   To show step-by-step changes from a starting value to an ending value -> Option C

4. Quick Check:

   Waterfall charts = stepwise changes [OK]

Hint: Waterfall charts show how values add or subtract stepwise [OK]

Common Mistakes:

• Confusing waterfall charts with line charts
• Thinking waterfall charts show data distribution
• Assuming waterfall charts compare unrelated categories

2. Which of the following is the correct way to set the bottom parameter in plt.bar() to create a waterfall effect?

easy

A. Use cumulative sums of previous bars as the bottom values

B. Set bottom to zero for all bars

C. Use random values for bottom to create spacing

D. Set bottom to the maximum value of the dataset

Solution

1. Step 1: Understand the bottom parameter in bar charts

   The bottom parameter sets where each bar starts vertically. For waterfall charts, bars start where the previous bar ended.

2. Step 2: Use cumulative sums for correct stacking

   By using cumulative sums of previous values, each bar stacks correctly to show stepwise changes.

3. Final Answer:

   Use cumulative sums of previous bars as the bottom values -> Option A

4. Quick Check:

   Bottom = cumulative sums for waterfall [OK]

Hint: Bottom bars stack by adding previous bar heights [OK]

Common Mistakes:

• Setting bottom always to zero causes overlapping bars
• Using random bottoms breaks the waterfall flow
• Setting bottom to max value misplaces bars

3. What will be the output of this code snippet creating a waterfall chart?

import matplotlib.pyplot as plt
values = [100, -30, 20, -10]
cumulative = [100, 70, 90, 80]
plt.bar(range(4), values, bottom=[0, 100, 70, 90], color=['green', 'red', 'green', 'red'])
plt.show()

medium

A. A waterfall chart showing increases and decreases starting at 100

B. A bar chart with all bars starting at zero

C. A stacked bar chart with bars overlapping incorrectly

D. An error because bottom list length does not match values

Solution

1. Step 1: Analyze the bottom parameter values

   The bottom list [0, 100, 70, 90] correctly positions each bar to start where the previous ended, matching cumulative sums.

2. Step 2: Check colors and values

   Positive values are green, negative are red, showing increases and decreases clearly in a waterfall style.

3. Final Answer:

   A waterfall chart showing increases and decreases starting at 100 -> Option A

4. Quick Check:

   Bottom aligns bars for waterfall effect [OK]

Hint: Check bottom aligns with cumulative sums for correct waterfall [OK]

Common Mistakes:

• Confusing bottom values causing bars to overlap
• Ignoring color coding for increases/decreases
• Assuming bars start all at zero

4. Identify the error in this waterfall chart code:

import matplotlib.pyplot as plt
values = [50, -20, 30]
cumulative = [50, 30, 60]
plt.bar(range(3), values, bottom=[0, 50, 60], color=['green', 'red', 'green'])
plt.show()

medium

A. The bottom list should be [0, 50, 80] because 30 + 50 = 80

B. The bottom list should be [0, 50, 30] to match cumulative sums

C. The color list length does not match values length

D. The values list contains negative numbers which is not allowed

Solution

1. Step 1: Analyze cumulative sums and bottoms

   The cumulative sums are [50, 30, 60]. The bottoms are [0, 50, 60]. The third bottom should be 30 (cumulative[1]), not 60.

2. Step 2: Identify the mismatch

   The bottom list does not correctly match the cumulative sums; it should be [0, 50, 30].

3. Final Answer:

   The bottom list should be [0, 50, 30] to match cumulative sums -> Option B

4. Quick Check:

   Bottoms = [0] + cumulative[:-1] [OK]

Hint: Bottom bars must start where previous bar ends [OK]

Common Mistakes:

• Using incorrect cumulative sums for bottom
• Mismatching color list length
• Assuming negative values are invalid

5. You want to create a waterfall chart for monthly profit changes: start at 200, then +50, -30, +20, -10. Which code snippet correctly calculates the bottoms for the bars?

hard

A. bottoms = [0, 50, 20, -10, 0]

B. bottoms = [200, 250, 220, 240, 230]

C. bottoms = [200, 0, 0, 0, 0]

D. bottoms = [0, 200, 250, 220, 240]

Solution

1. Step 1: Calculate cumulative sums stepwise

   Start at 200, then add +50 = 250, then -30 = 220, then +20 = 240, then -10 = 230.

2. Step 2: Determine bottoms for each bar

   The first bar starts at 0, second at 200, third at 250, fourth at 220, fifth at 240 to show stepwise changes.

3. Final Answer:

   bottoms = [0, 200, 250, 220, 240] -> Option D

4. Quick Check:

   Bottoms = previous cumulative sums [OK]

Hint: Bottoms = cumulative sums before each step [OK]

Common Mistakes:

• Starting bottoms at data values instead of zero
• Using incorrect cumulative sums
• Setting all bottoms to zero