All about All about GraphsGraphs

• About different types of graphs,

• How to draw them when you are doing your practical work,

• How to interpret the different shapes.

Learning Objectives

You should learn :

Drawing a graphWhen should I

draw a line-graph…?

…and when should

I draw a bar-graph…?

…and when should I draw a histogram

or circle graph?

Drawing a graphLook at the table of your results:

If this column has• Many continuous ranges

of values,use a histogram:

• Continuous values,use a line-graph:

• Fixed items that are usually not numerical,Use a bar graph:

• Percentage of any kind,Use a circle graph

Drawing a graphWhat is the best way to draw a line-graph?

5 steps in drawing a graph1. Choose simple scales.For example:1 large square = 1 meter (1 m)or1 large square = 2 m, or 5 m, or 10 mBut never choose an awkward scale, like 1 square = 3 m or 7 mChoose a scale that will make your graph use most of the space available.

5 steps in drawing a graph1. Choose simple scales.Put the dependent variable on the ‘y-axis’

and the independent variable on the ‘x-axis’

5 steps in drawing a graph2a. Plot the points

neatly.To mark the points we usually use an X

xx

xx

xx

Re-check each one before your next step.

Usually you need 5 or more points for the graph.

5 steps in drawing a graph2b. Alternative plotting.•To mark the points we can also use a dot in a circle .

Re-check each one before your next step.

•Make sure the dot is very

small and the circle has a diameter no greater than

2mm

5 steps in drawing a graph3. If the points form a straight

line……draw the best straight line through them

xx

xx

xx

Check that it looks the best straight line.

‘line of best fit’

5 steps in drawing a graph4. If the points form a curve……draw a free-hand curve of best fit

Do not join the points like a ‘dot-to-dot’.

5 steps in drawing a graph5. If a point is not on the line……use your apparatus to check this measurement again

You should ignore anomalous points.

This is called an anomalous point. x

x

xx

xx

5 steps in drawing a graphIn summary:1. Choose good scales,

with the dependent variable on the y-axis

2. Plot the points carefully3. Draw a line of best fit

using a ruler for a straight line graph,4. or draw free-hand for a curved graph5. Check anomalous points.

An example would be the height of a plant against time

A straight line graph:

Time/days

Heig

ht/c

m

A special case is when the straight line goes through the origin :

origin

In this case the two quantities are directly proportional.

If you think your graph should go through the origin, then draw it exactly through the origin.

Don’t forget to place both, the 0 for the x-axis and the 0 for the y-axis.

0 1 2 3 4 5 6 Time/days

Height/cm

543210

A curved graph, rising :

The dependent variable rises quickly at first and then more slowlyHere is an example:

Example : the volume of O2 produced in the catalase reaction against the time.

Volu

me

of O

2

/ml

Time/s

Eventually the reaction will stop producing O2 gas.

A curved graph, falling :

The dependent variable falls quickly at first

and then more slowlyHere is an example:

Example: The mass of starch left in the amylase reaction.

Mass of starch/g

Time/s

Starch Maltose

Amylase

• Know how to draw a line-graph correctly,

• Be able to give examples of graphs with different shapes,

• Be able to interpret graphs with different shapes.

Learning OutcomesYou should now: