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ALL ABOUT GRAPHS
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 to the left.
(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).
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.
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 (see diagrams to the left).
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:
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.
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