Lab 3 - Introduction to Graphics

Transcription

1 Lab 3 - Introduction to Graphics Spring 2018 Contents 1 Graphics Plots in one dimension Plots in two dimensions: two-way plots Saving and Exporting a Graph

2 1 Graphics 1.1 Plots in one dimension Graphs in Stata are usually done using the graph command + a type of graph (see help graph for an overview). However, you do not always need to specify graph Graphs can be created also using the Menu bar: GRAPHICS choose type of chart choose variables and options This approach may be very useful when you are familiarizing with Stata, since it allows you to learn the syntax of each graph commands and the options that each graph can take. Also, it s good practice to check the Stata help whenever you encounter a new command!! For illustration, let us use a dataset on wages; type: bcuse wage1, clear A box and whiskers plot gives a graphical depiction of the distribution of the variable and it is basically a graphical description of the command sum wage, detail graph box wage Figure 1: Average hourly earnings It is organized as follows: 2

3 o o <- outliers whiskers <- 75th percentile (upper hinge) box --- <- median <- 25th percentile (lower hinge) whiskers o <- outside value the shaded box contains the median, the 25th and the 75th percentile; the dots falling out of the whiskers are the outliers outliers are either very large or very small values in the distribution of the variable basically, most of your data should be within the whiskers NOTE: the box plot is very convenient to depict data and spot outliers You can also make plots for a variable across several categories. For example, the dataset includes both females and males: to compare wage across gender: graph box wage, by(female) Figure 2: Wage by gender 3

4 Customizing box plots Type graph hbox wage, by(female) to flip the graph horizontally. Consider the following command (all written in one line): graph box wage, by(female, title("hourly Wage by Sex") note("source: Wooldridge Datasets, 2000")) ysize(7) xsize(10) If there is a by(varlist) option, type comma, inside it and use title("text") to assign a title to the box plot. Type note("text") to add a note to your plot. Use ysize(#) and xsize(#) to adjust the dimensions along the y- and x-axes. There are many more customization options, consult help graph box to learn more. Pie chart: Show in a pie chart the breakdown of females and males in the dataset: graph pie, over(female) plabel(_all percent) Figure 3: Pie chart of females and males where plabel(...) specifies an option that labels should appear on the slice, and _all percent specifies that we want the relevant percentages printed on the graph. NOTE: with the pie plot, you can also specify a variable name graph pie varname, over(group), however the slices will now give you the sum of varname, rather than the number of observation in each group. To make things simple, we will ONLY use the graph pie, over(group) command, without specifying the varname. Keep in mind this distinction when you solve the problem sets. Customizing pie charts Consider the following command (all written in one line): graph pie wage, over(female) plabel(_all percent, size(*1.5) color(white)) legend(off) title("hourly Wage by Sex") note("source: Wooldridge Datasets, 2000") 4

5 As before, option plabel specifies how to label pie slice according to the formula plabel(which what, how) which specifies which slice to label. It can be _all or 1, 2, etc. what specifies what to label the slice with. It can be sum sum of variable, percent percent of sum, name identity, and text specified text. how specifies how the label is to look. You can use size to specify the size of the label and color to set the color. legend(off) disables the legend that is on the pie chart by default. title("text") assigns a text title to the chart. note("text") is used to include notes, for example sources of data. As before, use ysize(#) and xsize(#) to adjust the dimensions along the y- and x-axes. There are many more customization options, consult help graph pie to learn more. 1.2 Plots in two dimensions: two-way plots twoway is a family of plots, all of which fit on numeric y and x scales To produce a scatter plot, type: scatter wage educ Figure 4: Scatter plot of wage and education The scatter plot can be conveniently overlaid with lfit, that plots the sample regression function (OLS fitted values). Here is an example: twoway (scatter wage educ) (lfit wage educ) twoway is a family of plots for two variables, all of which fit on numeric y and x scales: the scatter plot is a type of twoway plot 5

6 Figure 5: Scatter plot of wage and education with a fitted line When you want to call a graph from the twoway family, a plot type must be specified. Note that for the whole family of twoway graphs, you can leave out graph to save typing. Moreover, for plotting scatter and line types, you can even leave out twoway. Thus the following commands are identical: graph twoway scatter wage educ twoway scatter wage educ scatter wage educ There is a number of other types of graphs in twoway. You can type help graph twoway for the full list. Two-way plots commands can be combined with if statements. To see the relationship (scatter plot) between wage and experience for women, for example, type: scatter wage exper if female==1 Figure 6: Scatter plot of wage and experience for women only Customizing scatter plots Consider the following command (all written in one line): 6

7 scatter wage educ, title("hourly Wage and Education") ytitle("hourly Wage") xtitle("education") ylabel(0(10)25) xlabel(0(1)18) msymbol(o) mcolor(red) msize(small) xsize(10) ysize(4) As before, use title("text") to change the title of the plot. Use xtitle("text") and ytitle("text") to change axes labels. Use xlabel(o(s)e) and ylabel(o(s)e) to set where an axis should start o, what step an axis should have e, and where it should end e. If there is a by(varlist) option, type comma, inside it and use title("text") to assign a title to the box plot. Use msymbol() to choose the shape of markers, it can be O, D, T, etc. Check help symbolstyle for more Use msybol(color) to customize the color of markers. Use msize(size) to customize the size of markers, it can be tiny, small, large, huge, etc. Check help markersizestyle for more. Type note("text") to add a note to your plot. As before, use ysize(#) and xsize(#) to adjust the dimensions along the y- and x-axes. There are many more customization options, consult help scatter to learn more. Bar chart: To create a plot of a descriptive statistic of a particular variable over categories. You can choose which descriptive statistic you want to look at: mean is the default. p1 means first percentile, p2 second, and so on; p50 means the same as median. count means the number of nonmissing values of the specified variable. If you want to look at average wage by number of dependents, type: graph bar wage, over(numdep) Figure 7: Average wage by number of dependents If you want to look at median wage by number of dependents, type: graph bar (p50) wage, over(numdep) Customization options are pretty much the same as in the scatter plot case. Check help graph bar for more information. 7

8 p 50 of wage Figure 8: Median wage by number of dependents Histogram: To plot the empirical distribution of one variable. To make a histogram for wage for nonwhite women, for example, type: histogram wage if female==1 & nonwhite==1, percent Figure 9: Wage histogram of nonwhite women Instead of option percent, you could use options fraction, frequency, density to transform y-axis to fractions, density, or frequencies. Customizing histograms Consider the following command: histogram wage if female==1 & nonwhite==1, percent start(0) width(1) Use start(#) to customize the starting point of the x-axis. 8

9 Use width(#) to customize the size/step of the bar(bin). One could also customize the number of bins as opposed to the width of the bin histogram wage if female==1 & nonwhite==1, percent start(0) bin(3) Note that bin(#) and width(#) cannot be combined. Titles, legend, x and y axes are customized the same as scatter plots. 1.3 Saving and Exporting a Graph To save a graph use the command:. graph save mygraph, replace graph save is the actual command, mygraph is the filename, you can choose any other name you like. The graph is saved in your Current Working directory unless you specify otherwise. Typing:. graph save "L:\EC2227\mygraph", replace will save it in the folder EC2227. (Otherwise just use the cd "L:\EC2227" command at the beginning and work in that folder throughout) If filename is specified without an extension,.gph (STATA format) is assumed. this may be inconvenient if you just want to look at a plot you saved without having to open STATA To specify another extension you can use the command graph export. For example:. graph export mygraph.png, replace will save.png file which can be opened by any photo viewer. graph export also supports.pdf 9