MANAGING OUR ENVIRONMENT
April 2009
General information about reading
graphs
If the data points make a straight line going from the
Box Plots
origin out to high x- and y-values, then the variables are
Graph data as a box representing statistical values. said to have a positive correlation. If the line goes from a
The bottom boundary of the box indicates the 25 th high-value on the y-axis down to a high-value on the x-
percentile or lower quartile of the data, the line within axis, the variables have a negative correlation.
the box shows the median and the top boundary of the
box indicates the 75th percentile or upper quartile.
Whiskers (error bars) above and below the box indicate
the 90th and 10th percentiles and the points show the
last sample value.
Bar Chart
A bar chart or bar graph is a chart with rectangular bars
with lengths proportional to the values that they
represent. Bar charts are used for comparing two or
more values. The bars can be horizontally or vertically
oriented. Sometimes a stretched graphic is used instead
of a solid bar. It is a visual display used to compare the
amount or frequency of occurrence of different
Scatter Plots characteristics of data and it is used to compare groups
Scatter plots are similar to line graphs in that they use of data.
horizontal and vertical axes to plot data points.
However, they have a very specific purpose. Scatter
plots show how much one variable is affected by
another. The relationship between two variables is
called their correlation. Scatter plots usually consist of
a large body of data. The closer the data points come
when plotted to making a straight line, the higher the
correlation between the two variables, or the stronger
the relationship.
Reading WaterQualityMatters
Graphs
Box Plots
The box plots presented in WaterQualityMatters
demonstrate the relationship, by contaminant, between
all monitoring sites in a defined area from the most
upstream site to the most downstream.. The red line
indicates the Proposed One Plan standard, therefore
showing how close/far the quality at that particular
monitoring site is to meeting what’s proposed. If the
box is mostly above the standard this indicates that
more than 75% of the time the standard is exceeded at
the site whereas, if the median line on the box plot is
close or equal to the proposed standard, it is exceeded
50% of the time. If the majority of the box is below the Bar Chart
standard, then the standard is being met most of the The bar chart represents a summary for the site,
time. comparing the four major contaminants we monitor at
each state of the environment monitoring location. It
is a simple overview of the state of the river at that
location that takes no account of what is causing any
issues. The horizontal lines are placed on the chart
to enable the user to relate the readings here to the
coloration of the management zone (or subzone)
presented on the maps.
Scatter Plot
The scatter plot presented on the Consent Impact page
demonstrates the relationship between the monitoring
sites located directly upstream and directly downstream
of the consent discharge location. When points are
closer to the diagonal line, it demonstrates that there is
little impact by the consent discharge on the river. The
further above the diagonal line the points get, the more
impact the discharge has on the river and the worse the
water quality is likely to be.
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