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Introduction Data Management Stata Graphics Programming

3. Stata Graphics

Stata has excellent graphic facilities, accessible through the graph command, see help graph for an overview. The most common graphs in statistics are X-Y plots showing points or lines. These are available in Stata through the twoway subcommand, which in turn has 31 sub-subcommands or plot types, the most important of which are scatter and line. We will also describe briefly bar plots, available through the bar subcommand, and other plot types.

Stata 10 introduced a graphics editor that can be used to modify a graph interactively. We do not recomment this practice, however, because it conflicts with the goals of documenting and ensuring reproducibility of all the steps in your research.

3.1 Scatterplots

In this section we will illustrate a few plots using the data on fertility decline first used in Section 2.1. To read the data from net-aware Stata type

infile str14 country setting effort change ///
    using http://data.princeton.edu/wws509/datasets/effort.raw

To whet your appetite, here's the plot that we will produce in this section:

3.1.1 A Simple Scatterplot

To produce a simple scatterplot of fertility change by social setting you use the command

graph twoway scatter change setting 

Note that you specify y first, then x. Stata labels the axes using the variable labels, if they are defined, or variable names if not. The command may be abbreviated to twoway scatter, or just scatter if that is the only plot on the graph. We will now add a few bells and whistles.

3.1.2 Fitted Lines

Suppose we want to show the fitted regression line as well. In some packages you would need to run a regression, compute the fitted line, and then plot it. Stata can do all that in one step using the lfit plot type. (There is also a qfit plot for quadratic fits.) This can be combined with the scatter plot by enclosing each sub-plot in parenthesis. (One can also combine plots using two horizontal bars ||, but I find the method using parentheses clearer.)

graph twoway (scatter setting effort) ///
             (lfit setting effort)

Now suppose we wanted to put confidence bands around the regression line. Stata can do this with the lfitci plot type, which draws the confidence region as a gray band. (There is also a qfitci band for quadratic fits.) Because the confidence band can obscure some points we draw the region first and the points later

graph twoway (lfitci setting effort) ///
             (scatter setting effort) 

Note that this command doesn't label the y-axis but uses a legend instead. You could specify a label for the y-axis using the ytitle() option, and omit the (rather obvious) legend using legend(off). Here we specify both as options to the twoway command. To make the option more obvious to the reader I put the comma at the start of a new line:

graph twoway (lfitci setting effort) ///
             (scatter setting effort) ///
           , ytitle("Fertility Decline") legend(off)

3.1.3 Labeling Points

There are many options that allow you to control the markers used for the points, including their shape and color, see help marker_options. It is also possible to label the points using text included in another variable, using the mlabel(varname) option. In the next step we add the country names to the plot:

graph twoway (lfitci change setting) ///
             (scatter change setting, mlabel(country) ) 

One slight problem with the labels is the overlap of Costa Rica and Trinidad Tobago (and to a lesser extent Panama and Nicaragua). We can solve this problem by specifying the position of the label relative to the marker using a 12-hour clock (so 12 is above, 3 is to the right, 6 is below and 9 is to the left) and the mlabv() option. We create a variable to hold the position set by default to 3 o'clock and then move Costa Rica to 9 o'clock and Trinidad Tobago to just a bit above that at 11 o'clock (we can also move Nicaragua and Panama up a bit, say to 2 o'clock):

gen pos=3
replace pos = 11 if country == "TrinidadTobago"
replace pos = 9 if country == "CostaRica"
replace pos = 2 if country == "Panama" | country == "Nicaragua"
The graph then looks as follows
graph twoway (lfitci change setting) ///
             (scatter change setting, mlabel(country) mlabv(pos) ) 

3.1.4 Titles, Legends and Captions

There are options that apply to all two-way graphs, including titles, labels, and legends. Stata graphs can have a title() and subtitle(), usually at the top, and a legend(), note() and caption(), usually at the bottom, type help title_options to learn more. Usually a title is all you need. Stata 11 allows text in graphs to include bold, italics, greek letters, mathematical symbols, and a choice of fonts, type help graph text to learn more.

Our final tweak to the graph will be to add a legend to specify the linear fit and 95% confidence interval, but not fertility decline itself. We do this using the order(2 "linear fit" 1 "95% CI") option of the legend to label the second and first items in that order. We also use ring(0) to move the legend inside the plotting area, and pos(5) to place the legend box near the 5 o'clock position. Our complete command is then

graph twoway (lfitci change setting) ///
             (scatter change setting, mlabel(country) mlabv(pos) ) ///
           , title("Fertility Decline by Social Setting") ///
             ytitle("Fertility Decline") ///
             legend(ring(0) pos(5) order(2 "linear fit" 1 "95% CI")) 

The result is the graph shown at the beginning of this section. (If your graph looks slightly different it's probably because we used different color schemes. I used a customized version of s2color. See 3.2.5 below for more information about schemes.)

3.1.5 Axis Scales and Labels

There are options that control the scaling and range of the axes, including xscale() and yscale(), which can be arithmetic, log, or reversed, type help axis_scale_options to learn more. Other options control the placing and labeling of major and minor ticks and labels, such as as xlabel(), xtick() and xmtick(), and similarly for the y-axis, see help axis_label_options. Usually the defaults are acceptable, but it's nice to know that you can change them.

3.2 Line Plots

I will illustrate line plots using data on U.S. life expectancy, available as one of the datasets shipped with Stata. (Try sysuse dir to see what else is available.)

sysuse uslifeexp

The idea is to plot life expectancy for white and black males over the 20th century. Again, to whet your appetite I'll start by showing you the final product, and then we will build the graph bit by bit.

5.2.1 A Simple Line Plot

The simplest plot uses all the defaults:

graph twoway line le_wmale le_bmale year 

If you are puzzled by the dip before 1920, Google "US life expectancy 1918". We could abbreviate the command to twoway line, or even line if that's all we are plotting. (This shortcut only works for scatter and line.)

The line plot allows you to specify more than one "y" variable, the order is y1, y2, ..., ym, x. In our example we specified two, corresponding to white and black life expectancy. Alternatively, we could have used two line plots: (line le_wmale year) (line le_bmale year).

5.2.2 Titles and Legends

The default graph is quite good, but the legend seems too wordy. We will move most of the information to the title and keep only ethnicity in the legend:

graph twoway line le_wmale le_bmale year ///
            , title("U.S. Life Expectancy") subtitle("Males") ///
              legend( order(1 "white" 2 "black") )

Here we used three options, which as usual in Stata go after a comma: title, subtitle and legend. The legend option has many sub options; we used order to list the keys and their labels, saying that the first line represented whites and the second blacks. To omit a key you just leave it out of the list. To add text without a matching key use a hyphen (or minus sign) for the key. There are many other legend options, see help legend_option to learn more.

We would like to use space a bit better by moving the legend inside the plot area, say around the 5 o'clock position, where improving life expectancy has left some spare room. As noted earlier we can move the legend inside the plotting area by using ring(0), the "inner circle", and place it near the 5 o'clock position using pos(5). Because these are legend sub-options they have to go inside legend():

graph twoway line le_wmale le_bmale year ///
           , title("U.S. Life Expectancy") subtitle("Males") ///
             legend( order(1 "white" 2 "black") ring(0) pos(5) )

5.2.3 Line Styles

I don't know about you, but I find hard to distinguish the default lines on the plot. Stata lets you control the line style in different ways. The clstyle() option lets you use a named style, such as foreground, grid, yxline, or p1-p15 for the styles used by lines 1 to 15, see help linestyle. This is useful if you want to pick your style elements from a scheme, as noted further below.

Alternatively, you can specify the three components of a style: the line pattern, width and color:

Here's how to specify blue for whites and red for blacks:

graph twoway (line le_wmale le_bmale year , clcolor(blue red) ) ///
           , title("U.S. Life Expectancy") subtitle("Males") ///
	     legend( order(1 "white" 2 "black") ring(0) pos(5)) 

Note that clcolor() is an option of the line plot, so we put parentheses round the line command and inserted it there.

5.2.4 Scale Options

It looks as if improvements in life expectancy slowed down a bit in the second half of the century. This can be better appreciated using a log scale, where a straight line would indicate a constant percent improvement. This is easily done using the axis options of the two-way command, see help axis_options, and in particular yscale(), which lets you choose arithmetic, log, or reversed scales. There's also a suboption range() to control the plotting range. Here I will specify the y-range as 25 to 80 to move the curves a bit up:

graph twoway (line le_wmale le_bmale year , clcolor(blue red) ) ///
    , title("U.S. Life Expectancy") subtitle("Males") ///
      legend( order(1 "white" 2 "black") ring(0) pos(5)) ///
      yscale(log range(25 80))

3.2.5 Graph Schemes

Stata uses schemes to control the appearance of graphs, see help scheme. You can set the default scheme to be used in all graphs with set scheme_name. You can also redisplay the (last) graph using a different scheme with graph display, scheme(scheme_name).

To see a list of available schemes type graph query, schemes. Try s2color for screen graphs, s1manual for the style used in the Stata manuals, and economist for the style used in The Economist. Using the latter we obtain the graph shown at the start of this section.

graph display, scheme(economist)

3.3 Managing Graphs

Stata keeps track of the last graph you have drawn, which is stored in memory, and calls it "Graph". You can actually keep more than one graph in memory if you use the name() option to name the graph when you create it. This is useful for combining graphs, type help graph combine to learn more. Note that graphs kept in memory disappear when you exit Stata, even if you save the data, unless you save the graph itself.

To save the current graph on disk using Stata's own format, type graph save filename. This command has two options, replace, which you need to use if the file already exists, and asis, which freezes the graph (including its current style) and then saves it. The default is to save the graph in a live format that can be edited in future sessions, for example by changing the scheme. After saving a graph in Stata format you can load it from the disk with the command graph use filename. (Note that graph save and graph use are analogous to save and use for Stata files.) Any graph stored in memory can be displayed using graph display. (You can also list, describe, rename, copy, or drop graphs stored in memory, type help graph_manipulation to learn more.)

If you plan to incorporate the graph in another document you will probably need to save it in a more portable format. Stata's command graph export filename can export the graph using a wide variety of vector or raster formats, which is usually understood from the file extension. Vector formats such as Windows metafile (wmf or emf) or Adobe's PostScript and its variants (ps, eps, pdf) contain essentially drawing instructions and are thus resolution independent, so they are best for inclusion in other documents where they may be resized. Raster formats such as Portable Network Graphics (png) save the image pixel by pixel using the current display resolution, and are best for inclusion in web pages.

You can also print a graph using graph print, or copy and paste it into a document using the Windows clipboard; to do this right click on the window containing the graph and then select copy from the context menu.


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