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#### 8.3.14 `central`

Plots the nominal central point of an area. This appears just like a normal marker plot, but can be used when the available geometry information is an area description (such as an STC-S string or an array of polygon vertices) rather than coordinate values such as an X,Y pair. The position plotted is the nominal center of the shape as determined by the plotting code; that may or may not correspond to the actual center.

Usage Overview:

       layerN=central shapeN=filled_circle|open_circle|... sizeN=<pixels>
                      shadingN=auto|flat|translucent|transparent|density|aux|weighted|paux|pweighted <shade-paramsN>
                      areaN=<area-expr>
                      areatypeN=POINT|CIRCLE|POLYGON|MOC-ASCII|UNIQ|STC-S|TFCAT
                      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 xmin=136.7 xmax=138.5 ymin=-5.7 ymax=-4.2
                         in=crism.fits icmd='select sensor_id==0x53'
                         area=s_region areatype=STC-S
                         layer1=area polymode1=fill shading1=density densemap1=heat
                         layer2=central shape2=fat_circle size2=3 color2=black

`areaN = <area-expr>` (String)
  Expression giving the geometry of a 2D region on the plot. It may be a string- or array-valued expression, and its interpretation depends on the value of the corresponding `areatype` parameter.

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

`areatypeN = POINT|CIRCLE|POLYGON|MOC-ASCII|UNIQ|STC-S|TFCAT` (AreaMapper)
  Selects the form in which the Area value for parameter `areaN` is supplied. Options are:
  - `POINT`: 2-element array (`x`,`y`)
  - `CIRCLE`: 3-element array (`x`, `y`, `r`)
  - `POLYGON`: 2n-element array (`x1`,`y1`, `x2`,`y2`,...); a `NaN`,`NaN` pair can be used to delimit distinct polygons.
  - `MOC-ASCII`: Region description using ASCII MOC syntax; see MOC 2.0 sec 4.3.2. Note there are currently a few issues with MOC plotting, especially for large pixels.
  - `UNIQ`: Region description representing a single HEALPix cell as defined by an UNIQ value, see MOC 2.0 sec 4.3.1.
  - `STC-S`: Region description using STC-S syntax; see TAP 1.0, section 6. Note there are some restrictions: `<frame>`, `<refpos>` and `<flavor>` metadata are ignored, polygon winding direction is ignored (small polygons are assumed) and the `INTERSECTION` and `NOT` constructions are not supported. The non-standard `MOC` construction is supported.
  - `TFCAT`: Time-Frequency region defined by the TFCat standard. Support is currently incomplete; holes in Polygons and MultiPolygons are not displayed correctly, single Points may not be displayed, and Coordinate Reference System information is ignored.
  If left blank, a guess will be taken depending on the data type of the value supplied for the `areaN` value.
`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 alternatively 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. A backslash character '`\`' at the end of a line joins it with the following line.

`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`]

`shadingN = auto|flat|translucent|transparent|density|aux|weighted|paux|pweighted <shade-paramsN>` (ShapeMode)
  Determines how plotted objects in layer N are coloured. This may be influenced by how many objects are plotted over each other as well as the values of other parameters. Available options (Section 8.4) are:
  - `auto`
  - `flat`
  - `translucent`
  - `transparent`
  - `density`
  - `aux`
  - `weighted`
  - `paux`
  - `pweighted`
  Each of these options comes with its own set of parameters to specify the details of how colouring is done.

  [Default: `auto`]

`shapeN = filled_circle|open_circle|...` (MarkerShape)
  Sets the shape of markers that are plotted at each position of the scatter plot.

  The available options are:

  - `filled_circle`
  - `open_circle`
  - `cross`
  - `x`
  - `open_square`
  - `open_diamond`
  - `open_triangle_up`
  - `open_triangle_down`
  - `fat_circle`
  - `fat_cross`
  - `fat_x`
  - `fat_square`
  - `fat_diamond`
  - `fat_triangle_up`
  - `fat_triangle_down`
  - `filled_square`
  - `filled_diamond`
  - `filled_triangle_up`
  - `filled_triangle_down`

  [Default: `filled_circle`]

`sizeN = <pixels>` (Integer)
  Size of the scatter plot markers. The unit is pixels, in most cases the marker is approximately twice the size of the supplied value.

  [Default: `1`]

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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