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  Wiggle Track Format (WIG)

The wiggle (WIG) format is for display of dense, continuous data such as GC percent, probability scores, and transcriptome data. Wiggle data elements must be equally sized. If you need to display continuous data that is sparse or contains elements of varying size, use the BedGraph format instead. If you have a very large data set and you would like to keep it on your own server, you should use the bigWig data format.

For speed and efficiency, wiggle data is compressed and stored internally in 128 unique bins. This compression means that there is a minor loss of precision when data is exported from a wiggle track (i.e., with output format "data points" or "bed format" within the table browser). The bedGraph format should be used if it is important to retain exact data when exporting.

See also: Selecting a graphing track data format.

General Structure

Wiggle format is line-oriented. For wiggle custom tracks, the first line must be a track definition line, which designates the track as a wiggle track and adds a number of options for controlling the default display.

Wiggle format is composed of declaration lines and data lines. There are two options for formatting wiggle data: variableStep and fixedStep. These formats were developed to allow the file to be written as compactly as possible.

  • variableStep is for data with irregular intervals between new data points and is the more commonly used wiggle format. It begins with a declaration line and is followed by two columns containing chromosome positions and data values:
      variableStep  chrom=chrN  [span=windowSize]
      chromStartA  dataValueA
      chromStartB  dataValueB
      ... etc ...  ... etc ...
    The declaration line starts with the word variableStep and is followed by a specification for a chromosome. The optional span parameter (default: span=1) allows data composed of contiguous runs of bases with the same data value to be specified more succinctly. The span begins at each chromosome position specified and indicates the number of bases that data value should cover. For example, this variableStep specification:
    variableStep chrom=chr2
    300701 12.5
    300702 12.5
    300703 12.5
    300704 12.5
    300705 12.5
    is equivalent to:
    variableStep chrom=chr2 span=5
    300701 12.5
    Both versions display a value of 12.5 at position 300701-300705 on chromosome 2.

  • fixedStep is for data with regular intervals between new data values and is the more compact wiggle format. It begins with a declaration line and is followed by a single column of data values:
      fixedStep  chrom=chrN  start=position  step=stepInterval  [span=windowSize]
      ... etc ...
    The declaration line starts with the word fixedStep and includes specifications for chromosome, start coordinate, and step size. The span specification has the same meaning as in variableStep format. For example, this fixedStep specification:
    fixedStep chrom=chr3 start=400601 step=100
    displays the values 11, 22, and 33 as single-base regions on chromosome 3 at positions 400601, 400701, and 400801, respectively. Adding span=5 to the declaration line:
    fixedStep chrom=chr3 start=400601 step=100 span=5
    causes the values 11, 22, and 33 to be displayed as 5-base regions on chromosome 3 at positions 400601-400605, 400701-400705, and 400801-400805, respectively.

    Note that for both variableStep and fixedStep formats, the same span must be used throughout the dataset. If no span is specified, the default span of 1 is used.

    Likewise for the fixedStep format, the same step size must be used throughout the dataset. If no step size is specified, the default step size of 1 is used.

Data Values

Wiggle track data values can be integer or real, positive or negative values. Chromosome positions are specified as 1-relative. For a chromosome of length N, the first position is 1 and the last position is N. Only positions specified have data. Positions not specified do not have data and will not be graphed. All positions specified in the input data must be in numerical order.

Parameters for custom wiggle track definition lines

All options are placed in a single line separated by spaces:
  track type=wiggle_0 name=track_label description=center_label
        visibility=display_mode color=r,g,b altColor=r,g,b
        priority=priority autoScale=on|off alwaysZero=on|off
        gridDefault=on|off maxHeightPixels=max:default:min
        graphType=bar|points viewLimits=lower:upper
        yLineMark=real-value yLineOnOff=on|off
        windowingFunction=maximum|mean|minimum smoothingWindow=off|2-16

(Note if you copy/paste the above example, you have to remove the carriage returns.)

The track type with version is REQUIRED, and it currently must be wiggle_0:
  type wiggle_0
The remaining values are OPTIONAL:
  name              <trackLabel>                         # default is "User Track"
  description       <centerLabel>                        # default is "User Supplied Track"
  visibility        <full|dense|hide>                    # default is hide (will also take numeric values 2|1|0)
  color             <RRR,GGG,BBB>                        # default is 255,255,255
  altColor          <RRR,GGG,BBB>                        # default is 128,128,128
  priority          <N>                                  # default is 100
  autoScale         <on|off>                             # default is on
  alwaysZero        <on|off>                             # default is off
  gridDefault       <on|off>                             # default is off
  maxHeightPixels   <max:default:min>                    # default is 128:128:11
  graphType         <bar|points>                         # default is bar
  viewLimits        <lower:upper>                        # default is range found in data
  viewLimitsMax     <lower:upper>                        # UNENFORCED: suggested bounds of viewLimits
  yLineMark         <real-value>                         # default is 0.0
  yLineOnOff        <on|off>                             # default is off
  windowingFunction <mean+whiskers|maximum|mean|minimum> # default is maximum
  smoothingWindow   <off|[2-16]>                         # default is off
  transformFunc     <NONE|LOG>                           # default is NONE
  wigColorBy        <bed table>                          # use colors in bed table for wiggle in overlapping regions
  spanList          <first>                              # sets spans to be first span in table
                    <s1,s2,s3...>                        # list of spans in the loaded table
                                                         # you can find the spans with the following SQL query:
                                                         # "SELECT span FROM <table> GROUP BY span"
                                                         # typically spanList is only 1:
                                                         # spanList 1
                                                         # rarely, it may be more:
                                                         # spanList 1,1000
                                                         # special efforts must be made to load extra spans
                                                         # into the table for special purposes
A functional description of these options can be seen in the Wiggle track configuration description. (Custom tracks do not have interactive configuration options.)


This example specifies 19 separate data points in two tracks in the region chr19:49,304,200-49,310,700. To view this example as a custom track in the Genome Browser, copy the text and paste it into the "Add Custom Tracks" text box.

browser position chr19:49304200-49310700
browser hide all
#	150 base wide bar graph at arbitrarily spaced positions,
#	threshold line drawn at y=11.76
#	autoScale off viewing range set to [0:25]
#	priority = 10 positions this as the first graph
#	Note, one-relative coordinate system in use for this format
track type=wiggle_0 name="variableStep" description="variableStep format" \
    visibility=full autoScale=off viewLimits=0.0:25.0 color=50,150,255 \
    yLineMark=11.76 yLineOnOff=on priority=10
variableStep chrom=chr19 span=150
49304701 10.0
49304901 12.5
49305401 15.0
49305601 17.5
49305901 20.0
49306081 17.5
49306301 15.0
49306691 12.5
49307871 10.0
#	200 base wide points graph at every 300 bases, 50 pixel high graph
#	autoScale off and viewing range set to [0:1000]
#	priority = 20 positions this as the second graph
#	Note, one-relative coordinate system in use for this format
track type=wiggle_0 name="fixedStep" description="fixedStep format" visibility=full \
    autoScale=off viewLimits=0:1000 color=0,200,100 maxHeightPixels=100:50:20 \
    graphType=points priority=20
fixedStep chrom=chr19 start=49307401 step=300 span=200

Caution for sparse variableStep data

The wiggle format was designed for quickly displaying data that is quite dense. The variableStep format, in particular, becomes very inefficient when there are only a few data points per 1,024 bases. If variableStep data points (i.e., chromStarts) are greater than about 100 bases apart, it is advisable to use BedGraph format.