You should always be critical, when data are represented for you in a chart. Data can be manipulated in a way that promotes a given idea or point of view that the sender might have.
Here is an example:
Tom’s parents provide him with a monthly allowance of 20 dollars. In order to convince his parents, that his allowance should be raised by 10 dollars, he makes a bar chart showing his monthly allowance before and after the increase. As response to that Tom's parents makes another chart showing the same increase, but as you can see below, Tom’s chart looks very different from his parents’ chart, thou they are representing the same data.
|Tom’s chart||The parents’ chart|
"Look, mom and dad, the red bar
shows my current allowance, and
the blue bar shows my allowance
after it has been raised. Look,
how small the change is!”
(Tom has made his chart, så that
the scale goes to 1000 dollars,
which is quite high up, when
dealing with amounts like 20 and
30 dollars, and that makes the
Change of 10 dollars looks very
"No Tom, take a look at our chart and
see how big the change is!"
(In the parents’ chart, the scale is
logarithmic, that is, the scale increases
more than an ordinary scale, causing
the heights of the two bars to appear
very different from each other.
To follow up on the story, Tom’s allowance is now 30 dollars per month.
In return, however, he must empty the dishwasher twice a week.
As you can see it’s very easy to make things look different from what they really are. It all depends on the way you choose to represent your data.
At the stock markets, for instance, an increase of the rate from 99.5 to 99.8 seems more dramatically and therefore more attractive, if the scale on the chart begins at 99.0 instead of zero.