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help for bihist
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Graph bihistograms: histograms for two categories shown in opposite directions 
> from the abscissa

    bihist varname [if] [in] [weight], by(varname) [{ frequency |fraction |
            density } name(name_option) tw(twoway_options) tw1(
            twoway_options)]

Description

    bihist makes a bihistogram, with frequencies (optionally
    fraction/density) of one variable by two categories of another variable
    shown above and below the abscissa (the x axis).  Which category's
    histogram is above the axis and which is below depends on the ordering of
    the by() variable.

    fweights, aweights, and pweights are allowed (at user's own risk).

Options 

    by is required. Options after a comma within the parentheses are passed
        to twoway_bar.

    frequency |fraction | density are as in histogram.

    name names the graph something other than "Graph"; see name_option.

    tw(twoway_options) passes options to twoway_bar. Note that options passed
        by default may be superseded in this way, for example y axis labels,
        but some added options may be ignored as a result of existing options
        (e.g. yscale will not override the range specified by the existing y
        axis labels).

    tw1(twoway_options) passes options to twoway_bar that are intended to
        apply only to the first by group (the lower set of bars).  See the
        second example below.

    Other options are passed to twoway__histogram_gen.

Examples

    sysuse auto, clear
    bihist mpg, by(foreign)

    sysuse nlsw88, clear
    bihist wage [fw=hours], by(married) frac
    la var married ""
    bihist wage [fw=hours], by(married) frac
    bihist wage [fw=hours], by(married) tw1(color(cyan)) tw(color(blue))

Author

    Austin Nichols
    austinnichols@gmail.com

Also see: 

 Manual: [R] graph twoway bar

 Online:  help for histogram, twoway bar; byhist (if installed; install)