Slope = rate of change. Steepest = fastest. Crossing = equal values. Widening gap = one is growing faster. Always read the y-axis before computing any rate.
Line graphs show how a variable changes over time. They are the go-to format for trends, growth rates, and turning points. The slope of a segment indicates the rate of change for that interval.
Key insight: The steepest segment = the period with the fastest rate of change. The flattest segment = the most stable period. These are the two most common question targets.
When two lines cross, the two values are equal at that point. Before the crossing: one is higher. After: the other is. GMAT often asks "when did A surpass B?" — the answer is the crossing point.
The vertical gap between two lines at any point = the difference in their values. A widening gap = A is growing faster than B. A narrowing gap = B is closing in on A.
The steepest segment shows the fastest rate of change, not the largest absolute value.
Don't project trends past the last data point unless the question explicitly asks for projection.
If different lines use different y-axes, you can't directly compare heights — only trends.
Two lines with similar slopes might have very different actual rates if their y-axes have different scales.
Line charts assume smooth change between points. Actual data may have been different — but for GMAT, interpolate linearly.
Lines often appear to cross between labeled data points. Interpolate to find the approximate crossing value.
A line generally trending up may still have one declining period. Read "overall" as the net direction.
The gap between two lines at a given time is the vertical difference at that x-coordinate, not the horizontal distance.
Area = cumulative quantity. The line height at one point = value at that moment. These are different questions.
A line that curves upward might show a constant percentage increase even though the absolute increases grow over time.
A line graph shows monthly revenue from Jan to Jun: J=$40K, F=$50K, M=$45K, A=$60K, May=$70K, Jun=$65K. In which month did revenue decline most?
A line graph shows two companies' stock prices from Q1 to Q4. Company A: Q1=$20, Q4=$30. Company B: Q1=$60, Q4=$80. Which company's stock showed higher percentage growth?
A line graph shows website traffic. The steepest upward slope occurs between March and April. The steepest downward slope is between August and September. What does this indicate?
Two lines on a graph cross at Year 5, where both show $50M. Before Year 5, Line A is above Line B. After Year 5, Line B is above Line A. What happened at Year 5?
A line graph shows annual profit from 2018 to 2023: 2018=$10M, 2019=$15M, 2020=$8M, 2021=$18M, 2022=$22M, 2023=$28M. The year with the largest year-over-year decline is:
A line graph shows Population (Y-axis, millions) vs. Year (X-axis). The line is concave up (curve opens upward). This indicates:
On a multi-line chart, Line X is always above Line Y. The vertical gap between them is widening over time. This means:
A line graph shows monthly temperatures. The line peaks in July (35°C) and troughs in January (5°C). A student says "average annual temperature = (35+5)/2 = 20°C." This is:
A line graph shows quarterly earnings from Q1 Y1 to Q4 Y3 (12 data points). Earnings in Q1 Y1 = $5M. Earnings in Q4 Y3 = $20M. Over the entire 3-year period, what was the average annual growth rate (approximate)?
A line graph shows two products' market share over 5 years. Product P starts at 45% and ends at 30%. Product Q starts at 25% and ends at 40%. Approximately when did their shares equalize?
Steepest upward slope = fastest growth period. Steepest downward = sharpest decline.
Before crossing: one line is above. After: the other is. The crossing is when they're equal.
If the gap between two lines increases, the upper line is growing faster in absolute terms.
Line graph questions test what's in the chart. Projecting future trends requires explicit justification from context.