Centering columns in the histogram
The
HWidth
variable controls the width of the
columns, with the initial starting value and
end value set by
HRng
. In the frequency
table on the previous page the interval width
was 10. By setting
HWidth
to 10 in the
PLOT SETUP
view as shown right we can
produce the graph shown below. Note the changes to
Xtick
and
Ytick
also.
Another use for the variables
HWidth
and
HRng
is to
re-orient the columns so that they are centered on the
data point. For example, the data set below produces
the graph to the right of it using
Auto Scale
.
You may have noticed that the columns are not centered around the data
points 1, 2 and 3. Instead the columns extend from 1..2, 2..3 and 3..4.
This bothers some people and can easily be fixed by changing the
HRng
. If you look at the
PLOT SETUP
values you will see that they
are currently set to 1 to 4. This is the default behavior for
Auto Scale
.
To center the columns you need only change the minimum value for
HRng
by subtracting a half column width giving 0.5. The results are
shown right and you can see that the columns are now centered as
required.
If you want to use linear interpolation to obtain
better values for the median and quartiles then
you can download an aplet from the author’s
website called “Grouped Data”. This will
calculate the values of the medians and
quartiles using linear interpolation and will also
allow calculations such as “What value will cut off the top 15% of
data?”.
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