Line graphs and polygraphs

Detail

What a line graph is

• A line graph is a chart where you mark points for each value and then join the points with a line.
• It is best for time-series data, which means data recorded at regular times (every year, every month or every census).
• Common Indian examples:
• monthly temperature and rainfall of a weather station
• population growth over the census decades

• birth rate and death rate over the years

• Why use a line? The slope of the line shows the trend at a glance:

• a line going up means the value is rising
• a line going down means it is falling
• a steep line means fast change
• a flat line means little or no change

How to draw a line graph (4 steps)

1. Round off the data. For example, write 2.218% as 2.22. Simpler numbers are easier to plot.
2. Put time on the X-axis (the flat, horizontal line). Put the values on the Y-axis (the upright, vertical line).
3. Choose a scale that fits the smallest and the largest value. If any value is negative, the Y-axis must start below zero. The zero line is then drawn higher up the page.
4. Plot the points and join them. Join them freehand to get a smooth line, or with a ruler to get straight pieces.

Feature	X-axis	Y-axis
What it shows	Time (years, months, census decades)	The value being measured (%, °C, mm, rate)
Starting point	The first year of the data	Zero, or a value below zero if the data has negative values

NCERT example: India's annual population growth rate, 1911–2011 (Table 3.1, Fig. 3.2)

• 1911: 0.56% a year.
• 1921: −0.30%. This is the only negative decade in the series. So the Y-axis has to go below zero.

• In real numbers, India had 772,117 fewer people in 1921 than in 1911. That is a fall of about −0.31% over the decade (1911–21) [4].

• 1971–81: the peak, at about 2.20–2.22% a year.

• 2011: 1.79% a year (NCERT figure).
• Census 2011 gives an average annual exponential growth rate of 1.64% for 2001–11. (Average annual exponential growth rate: the yearly growth rate that, if it compounded every year, would give the same total growth over the decade.)

• Over the whole decade 2001–11, India's population grew by 17.64%, and the population in 2011 was 1,210.19 million (Census 2011, provisional) [3].

• What the line shows: a dip below zero around 1921, a steep climb after 1921, a peak in 1971–81, and a slow fall after that.

Why growth fell in 1911–21

• Cause → process → result:
• Causes: the 1918 influenza pandemic (a world-wide spread of deadly flu), famines and epidemics such as plague and cholera.
• Process: many more people died. The crude death rate (deaths per 1,000 people in a year) rose from 42.6 in 1901–11 to 47.2 in 1911–21. The birth rate was high, but deaths were higher [4].

• Result: the population fell, and India's only negative decade appears on the graph.

• 1921 is called the "great divide" (also the "demographic divide") in India's population history.

• Before 1921, growth was stagnant or stationary (1901–21).
• After 1921, the population grew without a break [4].
• For the phases of population growth, see the india-population note.

What a polygraph is

• A polygraph is a line graph with two or more lines on the same axes. It lets you compare several variables at once, for example several states or places over the same years.
• Each line is drawn in its own line style or colour, so the reader can tell them apart:
• solid ( —— )
• broken ( - - - )
• dotted ( ······ )

• dot-dash ( -·-·- )

• A legend (key) explains which line stands for which variable.

• Limit: with too many lines, the graph gets crowded and hard to read. In practice, use a few lines that are clearly different from each other.

NCERT example: sex ratio of Delhi, Haryana and Uttar Pradesh, 1961–2011 (Table 3.2, Fig. 3.3)

• Sex ratio means the number of females for every 1,000 males.
• Delhi went from 785 in 1961 to 868 in 2011 (final Census 2011) [2]. (NCERT: 866, from the provisional 2011 figures.)

Census year	Delhi	Haryana	Uttar Pradesh	India
1951	768	871	908	946
1961	785	868	907	941
1971	801	867	876	930
1981	808	870	882	934
1991	827	865	876	927
2001	821	861	898	933
2011	868	879	912	943

Source: Office of the Registrar General, India, in MoSPI, Women & Men in India 2016 [2].

• How to read the polygraph:
• Delhi's line stays lowest in every census, but it rises the most. Delhi's sex ratio improved by 13.02% from 1951 to 2011 [2]. One reason is that men from other states come to work in the city, so there are more men than women.
• Haryana's line stays almost flat at about 860–880. Its lowest point is 861 in 2001 [2].
• Uttar Pradesh's line drops in 1971 (876) and then climbs to 912 in 2011 [2].
• All three lines dip in 2001 or earlier and then rise in 2011. India also rose, from 933 in 2001 to 943 in 2011 [2].

Line graph vs polygraph

Point	Line graph	Polygraph
Number of lines	One	Two or more
Purpose	Shows the trend of one variable over time	Compares several variables over the same time
NCERT example	India's annual growth rate, 1911–2011 (Fig. 3.2)	Sex ratio of Delhi, Haryana and UP, 1961–2011 (Fig. 3.3)
Line style	One line	Different line styles or colours, with a legend

Prelims Hooks

• A line graph is best for time-series data: temperature, rainfall, population growth, birth and death rates.
• In a line graph, time goes on the X-axis and values go on the Y-axis.
• If any value is negative, the scale must go below zero. For India's growth rate, this was needed only for 1911–21 (−0.30%).
• 1921 is the "great divide" in India's population history. Growth was stagnant in 1901–21 and steady after 1921 [4].
• India's annual growth rate peaked in 1971–81 (about 2.20–2.22%). The decadal growth for 2001–11 was 17.64% (Census 2011) [3].
• Trap: a polygraph shows more than two variables. The option "two variables only" is wrong.
• In a polygraph, lines are told apart by solid, broken, dotted and dot-dash styles or by colour.
• Sex ratio in 2011, from lowest to highest: Delhi (868) < Haryana (879) < Uttar Pradesh (912) [2]. (NCERT gives Delhi as 866.)
• Delhi's sex ratio rose from 785 in 1961 to 868 in 2011, the biggest gain of the three [2].

Mains Points

• Spotting turning points in population history (GS-I):
• A line graph of India's growth rate shows four phases clearly: a fall in 1911–21, a steep rise after 1921, a peak in 1971–81, and a fall after that.
• Each phase has a cause. Famines, epidemics and the 1918 influenza caused deaths before 1921. Better health care later brought down the death rate. After 1981, a falling birth rate brought growth down.

• This makes the line graph a quick visual aid for questions on the demographic transition (the change from high birth and death rates to low ones).

• Comparing regions at a glance:

• A polygraph puts several regions on the same time scale.
• For example, the sex-ratio lines show that migration by men looking for jobs keeps Delhi's sex ratio low.
• They also show the son preference in Haryana and the improvement after 2001 in all three areas [2].

• Such a graph strengthens answers on gender, migration and regional imbalance.

• Using graphs in answers and planning:

• Line graphs and polygraphs help in tracking indicators over time, such as sex ratio, growth rate, rainfall departure and temperature anomaly.
• They show whether a scheme has had an effect, for example sex-ratio trends after programmes for the girl child.
• A simple hand-drawn line graph in a GS-I answer can show a trend faster than words.