The Graph You Pick Decides What People Notice

It matters a great deal. A bar graph makes categories easy to compare, a line graph makes change over time obvious, and a dot plot shows where values bunch together — the same numbers, three different stories.

Choose the wrong one and the answer gets buried. This page covers everything in the CBSE Class 7 Mathematics chapter's second half: dot plots, bar and double bar graphs, line graphs, and how to justify your choice of graph.

It shows the shape of the data — where values crowd together, where they thin out, and which ones sit alone.

To draw one, mark the values along a horizontal number line and stack one dot above the line for each observation. Equal values stack into a column, so the taller the stack, the more often that value occurred.

Suppose a class records the number of books each of 15 students read in a month: 1, 2, 2, 3, 3, 3, 3, 4, 4, 5, 2, 3, 1, 4, 12.

The dots pile up at 3 and 4, thin out after 5, and leave one dot far out at 12. Three features come straight off the picture:

- a cluster at 2 to 4, where most students sit
- a gap between 5 and 12, with no values at all
- an outlier at 12, well away from the rest

Those three words are what questions ask you to use, so describe a dot plot with them rather than reading out every value.

The outlier is also a cue to check the data, not just report it. A 12 among single-digit answers might be genuine, or it might be a recording slip — and the dot plot is what made it visible in the first place.

A bar graph uses bars of equal width with equal gaps, and the height of each bar shows its value against a stated scale.

Choosing the scale is the part that needs care. If the largest value is 40 and your graph paper allows 8 cm of height, take 1 cm = 5 units, so the tallest bar is $\frac{40}{5}=8$ cm. A scale that leaves the tallest bar 2 cm high wastes the page and hides differences.

A double bar graph puts two bars side by side for each category, shaded differently, so two groups can be compared on the same scale.

Suppose a class survey of favourite subjects gives, for boys and girls in turn: Maths 12 and 9, Science 8 and 14, English 6 and 10.

Reading it, Science shows the widest gap between the two bars, and Maths is the only subject where the first bar is taller. Comparisons like those are the reason the graph exists — a single-bar graph of totals would have hidden all of them.

Two requirements are marked every time: both axes labelled with what they show, and a key naming which shading is which group. A double bar graph without a key cannot be read at all, however neatly it is drawn.

A line graph plots points against time and joins them, so the slope of the line tells you what is happening.

Time always goes on the horizontal axis, the changing quantity on the vertical. Plot each point, then join consecutive points with straight line segments.

Suppose a school notes the temperature at 6 am on five consecutive days: $18°C$, $20°C$, $23°C$, $22°C$, $26°C$.

Reading the graph: rising segments mean the temperature increased, the fall on day 4 stands out against the general climb, and the overall trend is upward, from $18°C$ to $26°C$.

The steepness carries information too. The jump from $23°C$ to $26°C$ in the last step is the steepest rise, so that is where the change was fastest.

You can also read between the plotted points if the quantity changes smoothly — a point halfway along a segment from $18°C$ to $20°C$ suggests about $19°C$ at the midpoint.

That only works for continuous quantities like temperature and height. Joining the tops of bars for favourite subjects would suggest a value "between Maths and Science", which means nothing — so line graphs belong to data that changes over time, not to separate categories.

Match the graph to the question, and the choice makes itself:

- Comparing separate categories — bar graph.
- Comparing two groups across the same categories — double bar graph.
- Change over time — line graph.
- Showing the spread of a small data set — dot plot.

So "which subject is most popular?" wants a bar graph, "are boys and girls choosing differently?" wants a double bar graph, "is attendance improving over the term?" wants a line graph, and "how spread out are the reading habits of this class?" wants a dot plot.

Answering the statistical question then means quoting the graph as evidence. Not "Science is popular", but Science has the tallest bar for girls at 14 students, against 8 boys, so the preference differs between the two groups.

A warning about scales, since it is examined: a vertical axis that does not start at zero exaggerates small differences. Two bars of 48 and 50 look nearly equal from zero, but dramatically different if the axis starts at 45 — the numbers are honest, the picture is not. Always check where the axis begins before trusting how big a gap looks.