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ALL ABOUT GRAPHS : GRAPH DRAWING

GRAPHS, WHY USE THEM?
This page is available as a pdf download

This can be best demonstrated by means of an example.

Time / min

Pulse rate / beats min-1

-5

65

-4

69

-3

75

-2

65

-1

70

0

71

10 minute run

0

156

1

110

2

85

3

50

4

65

5

90

6

66

7

56

8

81

9

71

10

62

11

80

12

79

13

68

14

70

15

67

16

70

17

66

18

72

19

71

20

69

 

The table opposite shows a typical response of pulse rate, measured at one minute intervals, before and after a period of exercise. Notice the table contains pure dimensionless numbers (e.g. 4 and not 4 min). The column headings contain the relevant information including the units.

Just by looking at the table you can see the pulse has increased after the 10 minute run and then returns to normal after about 10 minutes, as you might expect. Plotting a curve of these data, however, reveals an interesting phenomenon. The return to normal is not a smooth one. The pulse rate passes below its resting value several times. This overshoot is normal and it illustrates that the heart beat is under negative feedback control.

  • The curve gives the experimenter a global impression of the spread and the trends in these data, at a glance.
  • Any points which stand out as unusual are easily seen on the graph because their distance from the other points is much greater than the other data points.
  • A graph makes it easy to make estimates between measured points.

All this interpretation of data could have been determined from the list of readings in the table but it is a lot easier to see what is going on when the results are displayed in the form of a graph.

Pulse Rate of a male 16 year old before and after a 10 minute run.

RULES FOR GRAPH DRAWING

(1) Determine the dependant and independent variables

In an experiment the experimenter will determine a set of conditions. It may be a range of temperatures or pH values, or, more commonly, the experimenter may choose to observe the experiment proceeding at set intervals of time (seconds, days or even years). These are the independent variable and always go on the horizontal axis (x—axis or abscissa). The effect of the experimenter varying the independent variable is measured as the dependant variables (the part of the experiment under observation), this is always plotted on the vertical axis (y—axis or ordinate).

Thus in an experiment to determine the effect of temperature upon the activity of a particular enzyme the axis should be set up as shown below.


Like this


Not like this

(2) Note the units of measurement for each of the variables

In the example above the temperature is likely to be in degrees Celsius (°C) but some chemistry and physics texts will use degrees Absolute or Kelvin (K). [NB None metric units such as Fahrenheit (°F) should be avoided in science.]

It is important to indicate to your audience what you are actually measuring your variables in. From the above example again, enzyme activity is usually measured in the amount of product produced per unit time. For example:
g product/min or g product min-1.

The units of measurement are presented behind the label of the axis after an oblique line not in brackets. e.g. Temperature / °C

(3) The proportions of the axes

The area enclosed by the axes should be roughly square and not disproportionately exaggerated..

If you are drawing the graph by hand on a piece of graph paper remember to leave yourself enough margin to write in your axis labels.
(4) Analysing your data

Look at your data carefully and determine the highest and lowest values for each of the two variables.

Values should increase as they progress away from the origin. At best this should be 0. This, however, may not be necessary especially if it wastes space and you want to maintain a roughly square graph. Start from a convenient but carefully chosen origin.

(5) Mark the quantities on both axes and number them at regular intervals

There is a temptation to space the numbers irregularly because your data, especially the dependant variable data, is often irregularly spaced. The labels on the axes should be regularly spaced so that the axis functions as a scale bar for intermediate values.
(6) Plotting and drawing the graph. Smooth curves, straight lines or trend lines?

If you are drawing the graph by hand, each point is marked clearly and boldly using a sharp HB pencil. Mark the data points as crosses or circles rather than dots.

You may join the points with a smooth curve passing through the points if they fall in a clear sequence and you think that the fluctuations in the curve are significant features of the data. However, because of errors and variations, a trend line (or curve) drawn between the points is usually best. If you cannot predict what data you would get between the different data points then they should be joined by straight lines.

DO NOT extend the line beyond the first and last data points given.

(7) Label the axes clearly with the variables and units

The Effect of Nitrates on Algal Growth

(8) Giving the graph a title
The graph must have a title which should contain a brief description of what is being investigated. Other information which may go in the title, if available, includes: the date, place and name of experimenter or collector of the data. If there is more than one graph a reference number or letter is required. For example:

"Fig 2 A graph showing the change in testis weight throughout the year in the brown rat (Rattus rattus)"

IS BETTER THAN...

" A graph of testis weight against time" which is insufficient.

Underline or use bold type for your title,  it makes it stand out and is easier to find on the page.

(9) Plotting more than one graph on a set of axes.

Sometimes two or three sets of data (though rarely more) are plotted within the same set of axes. You must distinguish between them by using different
symbols (X, O , , etc) or lines (…………., ________, -----------, etc). Avoid using colour for materials which is going to be printed.

The percentage change in length of apple and potato strips soaked in sucrose solutions

Notice the key by the side of the graph which explains the symbols or lines. Do not write on the graph itself though labels and arrows may be useful.

You may wish to plot data from two different dependant variables together on one graph but the values may be so different you have to use two different scales. One axis can be placed on each side of the graph as shown below.

Variations in the levels of progesterone and oestradiol during the menstrual cycle of a woman

However, the situation gets too confusing if more variables are treated in this way on one graph.

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© Paul Billiet 2014