**Plotting
a Graph**
When plotting a line graph you should:
Use a *sharp* pencil and a 12"
ruler.

Draw the whole graph in pencil first
and then when you are happy with it label the axes in ink, add a title
in ink and if you wish go over your points in ink or in a fine-tipped
felt or gel-ink pen.

Do NOT go over the line
in ink.

You must:

- choose an appropriate
scale
(i) so that
the graph fills most of the page. It doesn't matter which way round
you position the graph paper

(ii) so that the divisions on the axes make it
easy to plot the points accurately. Choose factors of 2 or 5 NOT
3 or 7!

- Give the graph
a title that explains what the experiment
was about, not simply 'A graph of temperature against time'.... that
can be gleaned from the labels on the axes.... something like 'Melting
ice' explains what you were doing as you recorded temperature and time
readings.
- Label
the axes with the physical quantity
and the unit it was measured in. For example
'mass (kg)'
- Plot
the points accurately and clearly. The best way to mark a point
is to use a neat cross. If the line is then drawn so that it obliterates
the point you can still see where it is.
- Draw an appropriate
best fit line graph (NOT DOT-TO-DOT graphs)
to fit the data, This may be a straight line graph or a curve. Your
points are NOT perfect, they
are subject to experimental error... your line can only give an indication
of the trend that they follow. Your line should be smooth...
no 'bumps' or 'wiggles'! A straight line should be drawn with a ruler,
not freehand!

**Interpreting your graph**

- If your graph
gives you a straight line it shows that
the two physical quantities you plotted are proportional.
If the straight line goes through the origin
the graph indicates that they are directly proportional....
i.e. if you double one quantity the other will double too.
- Any points that
are well away from the line are called anomalies.
They are probably due to experimental error. You should try to think
of how these anomalies could have occurred or what you could do next
time to avoid them happening.
- The line you
have drawn can be used to make predictions.
You can draw a line parallel to one of the axes and then direct it towards
the other axis after it has reached your graph line of best fit. This
can be done from any value on either axis and allow you to predict what
a pair of values in the experiement would probably be. In an exam always
pencil in these lines (draw
them as dashed lines) to show the examiner how you reached your answer.

**Finding
the gradient of a straight line graph**

For additional information on graph plotting
at AS level