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help for _gmlabvpos                                           manual: [R] egen 
                                                                      dialog:  
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Egen-Function to generate a variable for mlabvpos()

        egen [type] newvar = mlabvpos(yvar xvar) [if exp] [in range] [, log
                  polynom(#) mat:rix( 5x5 matrix)

    whereby yvar is the name of a variable which is going to be plotted as
    Y-variable in a scatterplot and xvar is the name of a variable which
    forms the X-axis of that scatterplot.

Description

    _gmlabvpos is an attempt to automatically generate a variable for the
    clockpositions of marker labels in scatterplots. That is, the command
    generates a variable which can be filled into the scatter option
    mlabvpos().

    Note that the program does not attempt to prevent marker labels from
    overploting, which is quite likely in dataset with many observations. In
    such situations you might be better of in simply make randomized clock
    positions:

    . gen clock = int(uniform()*12)+1)

    The general idea behind _gmlabvpos is to pull the marker label away from
    the data-region. For example, marker symbols in the lower left edge of
    the data region are labeled at clock-position 7 or 8, and marker symbols
    in the upper right edge of the data region are labeled at clock-position
    1 or 2, etc.  More precisely, if you consider the following rectangle as
    the data-region of a scatterplot, than marker labels of symbols in the
    indicated area gets the following clock-position:

         +--------------+
         |11 12 12 12  1|
         |10 11 12  1  2| 
         | 9  9 12  3  3| 
         | 8  7  6  5  4| 
         | 7  6  6  6  5| 
         +--------------+ 

    If yvar and xvar are highly correlated, than the clock-positions are
    generated as follows (which is however the same general idea):

         +--------------+
         |      12  1  3|
         |   12 12  3  4| 
         |11 11 12  5  5| 
         |10  9  6  6   | 
         | 9  7  6      | 
         +--------------+ 

    To calculate the the clock-positions, Stata first categorize the x-axis
    into 5 equal sized intervals around the mean of xvar. Afterwards the
    residuals of a linear regression of yvar on xvar are categorized into 5
    equal sized intervals. Both categorized variables are than used to form
    the clockpositions according to the rule of the first table above.  The
    rule can be changed with the option matrix().

Options

    log is used, if you want to calculate the residuals from the regression
        of yvar on a logarthmic version of xvar. This might be useful if the
        scatter shows a strong curvilinar relationship.

    polynom(#) is used, if you want to calculate the residuals from the
        regression of yvar on polynoms of xvar. For example use
        {cmd:polynom(2) if the scatter shows a u-shaped relationship.

    matrix(#) is used to change the general rule for the plot-positions. The
        clock positions are specified by a 5x5 matrix, whereby the upper left
        cell refer to the clock position of marker labels in the upper left
        part of the data-region. etc.


Examples

    . egen clock = mlabvpos(mpg weight)
    . sc mpg weight, mlab(make) mlabvpos(clock)
    . egen clock2 = mlabvpos(mpg weight), matrix(11 1 12 11 1 \\ 10 2 12 10 2
        \\ 9 3 12 9 3 \\ 8 4 6 8 4 \\ 7 5 6 7 5)
    . sc mpg weight, mlab(make) mlabvpos(clock2)


Also see

 
    Online:  help for scatter,


Author

 
    Ulrich Kohler, WZB, kohler@wz-berlin.de