Broken Axis Redux

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Often when the data includes some extreme difference in measures or some outliers, the plot of the data points can get skewed due to the need to accommodate the extreme outliers.  The bulk of the observations get squeezed into a smaller region of the plot.  While this may be useful in some cases, often we want to allow the data to fill the plot region while still drawing the outliers.

BarIn such cases, a broken axis  allows us to do just this.  The previous article described how to use the Broken Axis feature first released with SAS 9.40M1.

On the right is a contrived example of a case where the response value for one of the categories (E) is very large as compared to the other values.  The large value causes the other values for categories "A", "B", "C" and "D" to be scaled down to where it is hard to make a good comparisons between these values.

BarBrokenAxisFull

The graph on the right uses the Broken Axis feature first released with SAS 9.4M1, using the code shown below.  The RANGES option on the YAXIS statement allows us to specify the data ranges we want to keep in the graph.

proc sgplot data=tallbar;
  vbar x / response=y nostatlabel;
  yaxis ranges=(min-44  384-max) values=(0 to 400 by 10);
run;

In this case, we have requested to keep the ranges from minimum to 44 and from 384 to maximum.  Note, the value "44" is selected to place the break between the ticks.  Also, the axis VALUES need to be provided to give the best results.  The same tick interval is preserved in all the axis ranges.  There can be more than one break, but on only one axis at a time.

ScatterBrokenAxisBracketWith SAS 9.40M1, the break was represented in the graph as the "Full Break" shown in the graph above.  This was useful when breaking plot elements like bars or needles that have a continuous nature, and span over large parts of the graph width or height.  However, this type of break was considered too distracting, and not preferred when breaking an axis for a scatter plot.

Based on such feedback from users, SAS 9.40M3 release includes the ability for us to request a lighter weight "Axis Break" as shown in the graph on the right.  The code is shown below.  Note the use of the AXISBREAK option inthe STYLEATTRS statement.  Here we have selected "Bracket", which breaks only the axis using the two short bars.  Click on the graph for a higher resolution view.

proc sgplot data=outOfRange;
  styleattrs axisbreak=bracket;
  reg x=x y=y / clm markerattrs=(size=5);
  yaxis ranges=(min-1.5 8.9-max) values=(0 to 10 by 0.2) valueshint;
run;

ScatterBrokenAxisSparkMany different types of break symbols can be used, such as the "Spark" shown in the graph on the right.  If the axis line is absent, then the symbol is not shown.

Note another interesting feature in the graph on the right.  The X and Y axis lines does not extend all the way to touch each other.  This provides another "look and feel" for the graph display, often preferred by some users. This is controlled by the AXISEXTENT option on the STYLEATTRS statement as shown below.  Note, the axis line extends to the data range only.  So, in this case, the data range goes beyond the extreme tick marks on the y-axis.

styleattrs axisbreak=spark axisextent=data ;

Full SAS9.40M3 Code: BrokenAxis 

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About Author

Sanjay Matange

Director, R&D

Sanjay Matange is R&D Director in the Data Visualization Division responsible for the development and support of the ODS Graphics system, including the Graph Template Language (GTL), Statistical Graphics (SG) procedures, ODS Graphics Designer and related software. Sanjay has co-authored a book on SG Procedures with SAS/PRESS.

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5 Comments

  1. Hi Sanjay,

    I was using the code above for scatter plot where I wanted to break x axis. and I was getting an error. 121 ods listing style=analysis;
    122 ods graphics / reset width=5in height=3in imagename='ScatterBrokenAxisBracket';
    123 proc sgplot data=esc ;
    124 where Analysis_Time_Point_Hours>0;
    125 styleattrs axisbreak=bracket;
    ---------
    22
    202
    ERROR 22-322: Syntax error, expecting one of the following: ;, DATACOLORS, DATACONTRASTCOLORS,
    DATALINEPATTERNS, DATASYMBOLS.
    ERROR 202-322: The option or parameter is not recognized and will be ignored.
    126 scatter x= time1/*Analysis_Time_Point_Hours*/ y=avg_conc /group=Actual_Treatment
    126! yerrorlower=lower yerrorupper=upper markerattrs=(symbol=CircleFilled );
    127 series x= time1/*Analysis_Time_Point_Hours*/ y=avg_conc /group=Actual_Treatment
    127! lineattrs=(thickness=2 pattern=solid );
    128 yaxis values=(0 to 60000 by 5000)
    129 label="Mean (+/- SD)" minor;
    130
    131 xaxis ranges= (min-350 650-1425) values=(0 to 1425 by 50) valueshint
    132 label="Scheduled Time Point (Hours)" ;
    133 title1 'Mean Plasma Concentration (ng/mL) by Scheduled Sampling Time(hours) for Escalation
    133! Cohorts, linear scale (PK Population)';
    134 title2 'DRAFT';
    135 *REFLINE 0 / TRANSPARENCY = 0.5 AXIS=Y;
    136 run;

    NOTE: The SAS System stopped processing this step because of errors.
    NOTE: PROCEDURE SGPLOT used (Total process time):
    real time 0.00 seconds
    cpu time 0.00 seconds

    Do you know why?

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  3. Stuart Cowburn on

    Hello Sanjay,

    Do you have any suggestions for implementing a broken axis command in PROC SGPANEL? As far as I'm aware, the 'ranges' option is not available in SGPANEL.

    Thanks.

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