A scatter plot (Chambers 1983) reveals relationships or
association between two variables. **The relationship between two variables is
called ***correlation*. A scatter plot usually consists 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. If the data points make a straight line going from the origin out
to high x- and y-values, then the variables are said to have a **positive
correlation**. If the line goes from a high-value on the y-axis down to a
high-value on the x-axis, the variables have a **negative correlation**.

**Strong linear correlation: **The closer the number is
to 1 or -1, the stronger the correlation, or the stronger the relationship
between the variables.

**Weak linear correlation: **The closer the number is to 0,
the weaker the correlation.

**Scatter plot matrix: ** Given a set of variables, the scatter plot matrix
contains all **the pair-wise scatter plots of the variables** on a single page in a
matrix format. The example generated by XmdvTool shows a 4x4
scatter plot matrix of the variables medhvalue (median house value), rooms (# of rooms),
bedrooms (# of bedrooms), and households (# of households).

Top row in the graph shows 1. the scatter plot of medhvalue and rooms,
2. the scatter plot of medhvalue and bedrooms, and
3. the scatter plot of medhvalue and households.
The second row in the graph shows 4. the scatter plot of rooms and bedrooms
and 5. the scatter plot of rooms and households.
The third row in the graph shows 6. the scatter plot of bedrooms and households.
**This scatter plot matrix shows that rooms, bedrooms, and households are highly correlated.
Note that when there is a high correlation between A and B and between A and C,
there is a high correlation between B and C.**

**Reference: **

Chambers, John, William Cleveland, Beat Kleiner, and Paul
Tukey, (1983), *Graphical Methods for Data Analysis*, Wadsworth.

http://www.mste.uiuc.edu/courses/ci330ms/youtsey/scatterinfo.html

http://www.itl.nist.gov/div898/handbook/eda/section3/scatterp.htm

http://gsbapp2.uchicago.edu/sas/sashtml/insight/chap5/sect3.htm