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#### 8.3.27 `histogram`

Plots a histogram.

Bin heights may optionally be weighted by the values of some additional coordinate, supplied using the `weight` parameter. In this case you can choose how these weights are combined in each bin using the `combine` parameter.

Various options are provided for configuring how the bar heights are calculated, but note that not all combinations of the available parameters will necessarily lead to meaningful visualisations.

Usage Overview:

       layerN=histogram colorN=<rrggbb>|red|blue|... transparencyN=0..1
                        binsizeN=+<width>|-<count> phaseN=<number>
                        combineN=sum|sum-per-unit|count|...
                        cumulativeN=none|forward|reverse
                        normaliseN=none|area|unit|maximum|height
                        barformN=open|filled|semi_filled|steps|semi_steps|spikes
                        thickN=<pixels> dashN=dot|dash|...|<a,b,...> xN=<num-expr>
                        weightN=<num-expr> inN=<table> ifmtN=<in-format>
                        istreamN=true|false icmdN=<cmds>

All the parameters listed here affect only the relevant layer, identified by the suffix `N`.

Example:

       stilts plot2plane layer1=histogram in1=rrlyrae.fits x1=p1

`barformN = open|filled|semi_filled|steps|semi_steps|spikes` (Form)
  How histogram bars are represented. Note that options using transparent colours may not render very faithfully to some vector formats like PDF and EPS.

  The available options are:

  - `open`
  - `filled`
  - `semi_filled`
  - `steps`
  - `semi_steps`
  - `spikes`

  [Default: `semi_filled`]

`binsizeN = +<width>|-<count>` (BinSizer)
  Configures the width of histogram bins. If the supplied string is a positive number, it is interpreted as a fixed width in the data coordinates of the X axis (if the X axis is logarithmic, the value is a fixed factor). If it is a negative number, then it will be interpreted as the approximate number of bins to display across the width of the plot (though an attempt is made to use only round numbers for bin widths).

  When setting this value graphically, you can use either the slider to adjust the bin count or the numeric entry field to fix the bin width.

  [Default: `-30`]

`colorN = <rrggbb>|red|blue|...` (Color)
  The color of plotted data, given by name or as a hexadecimal RGB value.

  The standard plotting colour names are `red`, `blue`, `green`, `grey`, `magenta`, `cyan`, `orange`, `pink`, `yellow`, `black`, `light_grey`, `white`. However, many other common colour names (too many to list here) are also understood. The list currently contains those colour names understood by most web browsers, from `AliceBlue` to `YellowGreen`, listed e.g. in the Extended color keywords section of the CSS3 standard.

  Alternatively, a six-digit hexadecimal number RRGGBB may be supplied, optionally prefixed by "`#`" or "`0x`", giving red, green and blue intensities, e.g. "`ff00ff`", "`#ff00ff`" or "`0xff00ff`" for magenta.

  [Default: `red`]

`combineN = sum|sum-per-unit|count|...` (Combiner)
  Defines how values contributing to the same bin are combined together to produce the value assigned to that bin, and hence its height. The combined values are those given by the `weight` coordinate, but if no weight is supplied, a weighting of unity is assumed.

  The available options are:

  - `sum`: the sum of all the combined values per bin
  - `sum-per-unit`: the sum of all the combined values per unit of bin size
  - `count`: the number of non-blank values per bin (weight is ignored)
  - `count-per-unit`: the number of non-blank values per unit of bin size (weight is ignored)
  - `mean`: the mean of the combined values
  - `median`: the median of the combined values (may be slow)
  - `min`: the minimum of all the combined values
  - `max`: the maximum of all the combined values
  - `stdev`: the sample standard deviation of the combined values
  - `hit`: 1 if any values present, NaN otherwise (weight is ignored)

  [Default: `sum`]

`cumulativeN = none|forward|reverse` (Cumulation)
  If set to `forward`/`reverse` the histogram bars plotted are calculated cumulatively; each bin includes the counts from all previous bins working up/down the independent axis.

  Note that setting cumulative plotting may not make much sense with some other parameter values, for instance averaging aggregation modes.

  For reasons of backward compatibility, the values `true` and `false` may be used as aliases for `forward` and `none`.

  The available options are:

  - `none`: The value plotted for each bin uses the samples accumulated in that bin.
  - `forward`: The value plotted for each bin uses the samples accumulated all the way from negative infinity to that bin.
  - `reverse`: The value plotted for each bin uses the samples accumulated all the way from positive infinity to that bin.

  [Default: `none`]

`dashN = dot|dash|...|<a,b,...>` (float[])
  Determines the dash pattern of the line drawn. If null (the default), the line is solid.

  Possible values for dashed lines are `dot`, `dash`, `longdash`, `dotdash`. You can alternatively supply a comma-separated list of on/off length values such as "`4,2,8,2`".

`icmdN = <cmds>` (ProcessingStep[])
  Specifies processing to be performed on the layer N input table as specified by parameter `inN`. The value of this parameter is one or more of the filter commands described in Section 6.1. If more than one is given, they must be separated by semicolon characters (";"). This parameter can be repeated multiple times on the same command line to build up a list of processing steps. The sequence of commands given in this way defines the processing pipeline which is performed on the table.

  Commands may alteratively be supplied in an external file, by using the indirection character '@'. Thus a value of "`@filename`" causes the file `filename` to be read for a list of filter commands to execute. The commands in the file may be separated by newline characters and/or semicolons, and lines which are blank or which start with a '`#`' character are ignored.

`ifmtN = <in-format>` (String)
  Specifies the format of the input table as specified by parameter `inN`. The known formats are listed in Section 5.1.1. This flag can be used if you know what format your table is in. If it has the special value `(auto)` (the default), then an attempt will be made to detect the format of the table automatically. This cannot always be done correctly however, in which case the program will exit with an error explaining which formats were attempted. This parameter is ignored for scheme-specified tables.

  [Default: `(auto)`]

`inN = <table>` (StarTable)
  The location of the input table. This may take one of the following forms:
  - A filename.
  - A URL.
  - The special value "`-`", meaning standard input. In this case the input format must be given explicitly using the `ifmtN` parameter. Note that not all formats can be streamed in this way.
  - A scheme specification of the form `:<scheme-name>:<scheme-args>`.
  - A system command line with either a "`<`" character at the start, or a "`|`" character at the end ("`<syscmd`" or "`syscmd|`"). This executes the given pipeline and reads from its standard output. This will probably only work on unix-like systems.
  In any case, compressed data in one of the supported compression formats (gzip, Unix compress or bzip2) will be decompressed transparently.
`istreamN = true|false` (Boolean)
  If set true, the input table specified by the `inN` parameter will be read as a stream. It is necessary to give the `ifmtN` parameter in this case. Depending on the required operations and processing mode, this may cause the read to fail (sometimes it is necessary to read the table more than once). It is not normally necessary to set this flag; in most cases the data will be streamed automatically if that is the best thing to do. However it can sometimes result in less resource usage when processing large files in certain formats (such as VOTable). This parameter is ignored for scheme-specified tables.

  [Default: `false`]

`normaliseN = none|area|unit|maximum|height` (Normalisation)
  Defines how, if at all, the bars of histogram-like plots are normalised or otherwise scaled vertically.

  Note that some of the normalisation options may not make much sense with some other parameter values, for instance averaging aggregation modes.

  The available options are:

  - `none`: No normalisation is performed.
  - `area`: The total area of histogram bars is normalised to unity. For cumulative plots, this behaves like `height`.
  - `unit`: Histogram bars are scaled by the inverse of the bin width in data units. For cumulative plots, this behaves like `none`.
  - `maximum`: The height of the tallest histogram bar is normalised to unity. For cumulative plots, this behaves like `height`.
  - `height`: The total height of histogram bars is normalised to unity.

  [Default: `none`]

`phaseN = <number>` (Double)
  Controls where the horizontal zero point for binning is set. For instance if your bin size is 1, this value controls whether bin boundaries are at 0, 1, 2, .. or 0.5, 1.5, 2.5, ... etc.

  A value of 0 (or any integer) will result in a bin boundary at X=0 (linear X axis) or X=1 (logarithmic X axis). A fractional value will give a bin boundary at that value multiplied by the bin width.

  [Default: `0`]

`thickN = <pixels>` (Integer)
  Thickness of plotted line in pixels.

  [Default: `2`]

`transparencyN = 0..1` (Double)
  Transparency with which components are plotted, in the range 0 (opaque) to 1 (invisible). The value is 1-alpha.

  [Default: `0`]

`weightN = <num-expr>` (String)
  Weighting of data points. If supplied, each point contributes a value to the histogram equal to the data value multiplied by this coordinate. If not supplied, the effect is the same as supplying a fixed value of one.

  The value is a numeric algebraic expression based on column names as described in Section 10.

`xN = <num-expr>` (String)
  Horizontal coordinate.

  The value is a numeric algebraic expression based on column names as described in Section 10.

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STILTS - Starlink Tables Infrastructure Library Tool Set
Starlink User Note256
STILTS web page: http://www.starlink.ac.uk/stilts/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk