Stata 15 help for twoway

[G-2] graph twoway -- Twoway graphs

Syntax

[graph] twoway plot [if] [in] [, twoway_options]

where the syntax of plot is

[(] plottype varlist ..., options [)] [||]

plottype Description ------------------------------------------------------------------------- scatter scatterplot line line plot connected connected-line plot scatteri scatter with immediate arguments

area line plot with shading bar bar plot spike spike plot dropline dropline plot dot dot plot

rarea range plot with area shading rbar range plot with bars rspike range plot with spikes rcap range plot with capped spikes rcapsym range plot with spikes capped with symbols rscatter range plot with markers rline range plot with lines rconnected range plot with lines and markers

pcspike paired-coordinate plot with spikes pccapsym paired-coordinate plot with spikes capped with symbols pcarrow paired-coordinate plot with arrows pcbarrow paired-coordinate plot with arrows having two heads pcscatter paired-coordinate plot with markers pci pcspike with immediate arguments pcarrowi pcarrow with immediate arguments

tsline time-series plot tsrline time-series range plot

contour contour plot with filled areas contourline contour lines plot

mband median-band line plot mspline spline line plot lowess LOWESS line plot lfit linear prediction plot qfit quadratic prediction plot fpfit fractional polynomial plot lfitci linear prediction plot with CIs qfitci quadratic prediction plot with CIs fpfitci fractional polynomial plot with CIs

function line plot of function histogram histogram plot kdensity kernel density plot lpoly local polynomial smooth plot lpolyci local polynomial smooth plot with CIs -------------------------------------------------------------------------

The leading graph is optional. If the first (or only) plot is scatter, you may omit twoway as well, and then the syntax is

scatter ... [, scatter_options] [ || plot [plot [...]]]

and the same applies to line. The other plottypes must be preceded by twoway.

Regardless of how the command is specified, twoway_options may be specified among the scatter_options, line_options, etc., and they will be treated just as if they were specified among the twoway_options of the graph twoway command.

Menu

Graphics > Twoway graph (scatter, line, etc.)

Description

twoway is a family of plots, all of which fit on numeric y and x scales.

Remarks

Remarks are presented under the following headings:

Definition Syntax Multiple if and in restrictions twoway and plot options

Definition

Twoway graphs show the relationship between numeric data. Say that we have data on life expectancy in the United States between 1900 and 1940:

. sysuse uslifeexp2

. list in 1/8 +-------------+ | year le | |-------------| 1. | 1900 47.3 | 2. | 1901 49.1 | 3. | 1902 51.5 | 4. | 1903 50.5 | 5. | 1904 47.6 | |-------------| 6. | 1905 48.7 | 7. | 1906 48.7 | 8. | 1907 47.6 | +-------------+

We could graph these data as a twoway scatterplot,

. twoway scatter le year (click to run)

or we could graph these data as a twoway line plot,

. twoway line le year (click to run)

or we could graph these data as a twoway connected plot, marking both the points and connecting them with straight lines,

. twoway connected le year (click to run)

or we could graph these data as a scatterplot and put on top of that the prediction from a linear regression of le on year,

. twoway (scatter le year) (lfit le year) (click to run)

or we could graph these data in many other ways.

These all are examples of twoway graphs. What distinguishes a twoway graph is that it fits onto numeric y and x axes.

Each of what we produced above is called a graph. What appeared in the graphs are called plots. In the first graph, the plottype was a scatter; in the second, the plottype was a line; in the third, the plottype was connected; and in the fourth, there were two plots: a scatter combined with a line plot of a linear fit.

twoway provides many different plottypes. Some, such as scatter and line, simply render the data in different ways. Others, such as lfit, transform the data and render that. And still others, such as function, actually make up data to be rendered. This last class makes it easy to overlay y=x lines or y=f(x) functions on your graphs.

By the way, in case you are wondering, there are no errors in the above data. In 1918, there was an outbreak of influenza known as the 1918 Influenza Pandemic, which in the United States, was the worst epidemic ever known and which killed more citizens than all combat deaths of the 20th century.

Syntax

If we want to graph y1 versus x and y2 versus x, the formal way to type this is

. graph twoway (scatter y1 x) (scatter y2 x)

If we wanted y1 versus x plotted with solid circles and y2 versus x plotted with hollow circles, formally we would type

. graph twoway (scatter y1 x, ms(O)) (scatter y2 x, ms(Oh))

If we wanted y1 versus x plotted with solid circles and wanted a line graph for y2 versus x, formally we would type

. graph twoway (scatter y1 x, ms(O)) (line y2 x, sort)

The sort option is included under the assumption that the data are not already sorted by x.

We have shown the formal way to type each of our requests, but few people would type that. First, most users omit the graph:

. twoway (scatter y1 x) (scatter y2 x) . twoway (scatter y1 x, ms(O)) (scatter y2 x, ms(Oh)) . twoway (scatter y1 x, ms(O)) (line y2 x, sort)

Second, most people use the ||-separator notation rather than the ()-binding notation:

. twoway scatter y1 x || scatter y2 x . twoway scatter y1 x, ms(O) || scatter y2 x, ms(Oh) . twoway scatter y1 x, ms(O) || line y2 x, sort

Third, most people now omit the twoway:

. scatter y1 x || scatter y2 x . scatter y1 x, ms(O) || scatter y2 x, ms(Oh) . scatter y1 x, ms(O) || line y2 x, sort

And finally, most people quickly realize that scatter allows us to plot more than one y variable against the same x variable:

. scatter y1 y2 x . scatter y1 y2 x, ms(O Oh) . scatter y1 x, ms(O) || line y2 x, sort

The third example did not change: in that example, we are combining a scatterplot and a line plot. Actually, in this particular case, there is a way we can combine that, too:

. scatter y1 y2 x, ms(O i) connect(. l)

That we can combine scatter and line just happens to be an oddity of the examples we picked. It is important to understand that there is nothing wrong with any of the above ways of typing our request, and sometimes the wordier syntaxes are the only way to obtain what we want. If we wanted to graph y1 versus x1 and y2 versus x2, the only way to type that is

. scatter y1 x1 || scatter y2 x2

or to type the equivalent in one of the wordier syntaxes above it. We have to do this because scatter (see [G-2] graph twoway scatter) draws a scatterplot against one x variable. Therefore, if we want two different x variables, we need two different scatters.

In any case, we will often refer to the graph twoway command, even though, when we give the command, we will seldom type the graph, and mostly, we will not type the twoway either.

Multiple if and in restrictions

Each plot may have its own if exp and in range restrictions:

. twoway (scatter mpg weight if foreign, msymbol(O)) (scatter mpg weight if !foreign, msymbol(Oh))

Multiple plots in one graph twoway command draw one graph with multiple things plotted in it. The above will produce a scatter of mpg versus weight for foreign cars (making the points with solid circles) and a scatter of mpg versus weight for domestic cars (using hollow circles).

Also, the graph twoway command itself can have if exp and in range restrictions:

. twoway (scatter mpg weight if foreign, msymbol(O)) (scatter mpg weight if !foreign, msymbol(Oh)) if mpg>20

The if mpg>20 restriction will apply to both scatters.

We have chosen to show these two examples with the ()-binding notation because it makes the scope of each if exp so clear. In ||-separator notation, the commands would read

. twoway scatter mpg weight if foreign, msymbol(O) || scatter mpg weight if !foreign, msymbol(Oh)

and

. twoway scatter mpg weight if foreign, msymbol(O) || scatter mpg weight if !foreign, msymbol(Oh) || if mpg>20

or even

. scatter mpg weight if foreign, msymbol(O) || scatter mpg weight if !foreign, msymbol(Oh)

and

. scatter mpg weight if foreign, msymbol(O) || scatter mpg weight if !foreign, msymbol(Oh) || if mpg>20

We may specify graph twoway restrictions only, of course:

. twoway (scatter mpg weight) (lfit mpg weight) if !foreign

. scatter mpg weight || lfit mpg weight || if !foreign

twoway and plot options

graph twoway allows options, and the individual plots allow options. For instance, graph twoway allows the saving() option, and [G-2] graph twoway scatter allows the msymbol() option, which specifies the marker symbol to be used. Nevertheless, we do not have to keep track of which option belongs to which. If we type

. scatter mpg weight, saving(mygraph) msymbol(Oh)

the results will be the same as if we more formally typed

. twoway (scatter mpg weight, msymbol(Oh)), saving(mygraph)

Similarly, we could type

. scatter mpg weight, msymbol(Oh) || lfit mpg weight, saving(mygraph) or . scatter mpg weight, msymbol(Oh) saving(mygraph) || lfit mpg weight

and, either way, the results would be the same as if we typed

. twoway (scatter mpg weight, msymbol(Oh)) (lfit mpg weight), saving(mygraph)

We may specify a graph twoway option "too deeply", but we cannot go the other way. The following is an error:

. scatter mpg weight || lfit mpg weight ||, msymbol(Oh) saving(mygraph)

It is an error because we specified a scatter option where only a graph twoway option may be specified, and given what we typed, there is insufficient information for graph twoway to determine for which plot we meant the msymbol() option. Even when there is sufficient information (say that option msymbol() were not allowed by lfit), it would still be an error. graph twoway can reach in and pull out its options, but it cannot take from its options and distribute them back to the individual plots.


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