Create heat maps with PROC SGPLOT

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When SAS 9.4m3 was released last month (including SAS/STAT and SAS/IML 14.1), I was happy to see that a HEATMAP statement had been added to the SGPLOT procedure. Although heat maps in the SAS/IML language have been available for several releases, you previously had to use the Graph Template Language (GTL) to create a customized heat map in Base SAS.

Create heat maps in SAS with PROC SGPLOT #DataViz Click To Tweet

To illustrate using the HEATMAP statement, consider the following counts for patients in a medical study, classified by ordinal variables that represent the patients' weight and smoking status. The data and the techniques used to create the table are discussed in the article "How to order categories in a two-way table with PROC FREQ." You can download the complete SAS program that is used to create the table and graphs in this analysis.

t_orderfreq2

The categories and counts are contained in a data set called FreqOut, which was created by the FREQ procedure. The data set is arranged in "list form" (or "long form") as follows:

t_heatmap1

A basic heat map of a frequency table

The following statements create a basic heat map of the frequencies for the 15 cells of the table:

proc sgplot data=FreqOut;
   heatmap x=Weight_Cat y=Smoking_Cat / freq=Count 
         discretex discretey
         colormodel=TwoColorRamp outline;
run;
heatmapstmt1

On the HEATMAP statement, the DISCRETEX and DISCRETEY options are used because the underlying variables are numeric; a user-defined format is used to render the numbers as "Underweight," "Normal," "Overweight," and so forth. A two-color gradient ramp is good for showing counts. The default color ramp has three colors (blue, white, and red), which is good for visualizing deviations from a reference value.

The heat map makes it visually clear that most patients in the study are overweight non-smokers. Other categories with many patients include the overweight heavy-smokers, the normal-weight non-smokers, and the normal-weight heavy-smokers. Underweight patients (regardless of smoking status) are relatively rare.

Overlaying text on a heat map

Although this same heat map can be created in SAS/IML by using the HEATMAPCONT subroutine, using PROC SGPLOT makes it easy to overlay other plot types and change properties of the graph. For example, you might want to overlay the counts of each cell to create a color-coded table. You can easily add the TEXT statement to the previous SGPLOT statements. If you add the counts, you don't need to include the gradient color ramp. Furthermore, to make the heat map look more like the table, you can add the REVERSE option to the YAXIS statement. Lastly, the category labels make it clear that the variables are related to smoking and weight, so you can suppress the variable names. The following statements implement these revisions and create a new heat map:

proc sgplot data=FreqOut noautolegend;
   heatmap x=Weight_Cat y=Smoking_Cat / freq=Count 
         discretex discretey
         colormodel=TwoColorRamp outline;
   text x=Weight_Cat y=Smoking_Cat text=Count / textattrs=(size=16pt);
   yaxis display=(nolabel) reverse;
   xaxis display=(nolabel);
run;
heatmapstmt2

I'm excited by the HEATMAP statement in PROC SGPLOT. I think it is a great addition to one of my favorite SAS procedures.

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About Author

Rick Wicklin

Distinguished Researcher in Computational Statistics

Rick Wicklin, PhD, is a distinguished researcher in computational statistics at SAS and is a principal developer of PROC IML and SAS/IML Studio. His areas of expertise include computational statistics, simulation, statistical graphics, and modern methods in statistical data analysis. Rick is author of the books Statistical Programming with SAS/IML Software and Simulating Data with SAS.

10 Comments

  1. Michelle Homes

    Great enhancements to SAS' data visualization portfolio! I really like the text overlay providing the figures to the visual color "heat" without having to hover over for tool tips to compare.

    Thanks for sharing Rick.

  2. Hi,

    I was wondering if you can offer some insight. I tried to generate a heatmap with the code presented below

    proc sgplot data=work.nwn2
    heatmap x=trt y= response / freq=count
    discretex discretey
    colormodel=TwoColorRamp outline;
    run;

    I got the error presented below

    heatmap x=trt y= response / freq=count
    _______
    22
    76
    ERROR 22-322: Syntax error, expecting one of the following: ;, (, ASPECT, CYCLEATTRS, DATA, DATTRMAP, DESCRIPTION, NOAUTOLEGEND,
    NOBORDER, NOCYCLEATTRS, NOOPAQUE, NOSUBPIXEL, NOWALL, OPAQUE, PAD, PCTLEVEL, PCTNDEC, RATTRMAP, SGANNO, SUBPIXEL,
    TMPLOUT, UNIFORM.

    regards,
    Sen

  3. Pingback: Twelve posts from 2015 that deserve a second look - The DO Loop

  4. Cassandra M Plank on

    Is it possible to overlay text of the significance such as "n.s." for non-significance and "*" or "**" for level of significance instead of overlaying data numbers?

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