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#### 8.3.7 `xycorr`

Plots an error ellipse (or rectangle or other similar figure) defined by errors in the X and Y directions, and a correlation between the two errors.

The supplied correlation is a dimensionless value in the range -1..+1 and is equal to the covariance divided by the product of the X and Y errors. The covariance matrix is thus:

        [  xerr*xerr         xerr*yerr*xycorr  ]
        [  xerr*yerr*xycorr  yerr*yerr         ]

In some cases the supplied data values give the actual extents in data coordinates for the plotted ellipses but sometimes the data is on a different scale or in different units to the positional coordinates. As a convenience for this case, the plotter can optionally scale the magnitudes of all the ellipses to make them a reasonable size on the plot, so by default the largest ones are a few tens of pixels long. This auto-scaling is turned off by default, but it can be activated with the `autoscale` option. Whether autoscaling is on or off, the `scale` option can be used to apply a fixed scaling factor.

This plot type is suitable for use with the `<x>_error` and `<x>_<y>_corr` columns in the Gaia source catalogue.

Usage Overview:

       layerN=xycorr ellipseN=ellipse|crosshair_ellipse|... thickN=<int-value>
                     scaleN=<factor> autoscaleN=true|false
                     shadingN=auto|flat|translucent|transparent|density|aux|weighted|paux|pweighted <shade-paramsN>
                     xN=<num-expr> yN=<num-expr> xerrN=<num-expr> yerrN=<num-expr>
                     xycorrN=<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 in=tgas_source.fits icmd='select skyDistanceDegrees(ra,dec,56.9,23.9)<0.4'
                         x=pmra y=pmdec
                         layer1=mark
                         xerrhi2=pmra_error yerrhi2=pmdec_error
                         color2=cyan shading2=transparent
                         layer2a=xyerror errorbar2a=filled_rectangle opaque2a=10
                         layer2b=xyerror errorbar2b=crosshair_rectangle opaque2b=4
                         layer3=xycorr autoscale3=false
                         xerr3=pmra_error yerr3=pmdec_error xycorr3=pmra_pmdec_corr
                         ellipse3=crosshair_ellipse
                         aspect=1
                         xmin=17 xmax=24 ymin=-48 ymax=-42

`autoscaleN = true|false` (Boolean)
  Determines whether the default size of variable-sized markers like vectors and ellipses are automatically scaled to have a sensible size. If true, then the sizes of all the plotted markers are examined, and some dynamically calculated factor is applied to them all to make them a sensible size (by default, the largest ones will be a few tens of pixels). If false, the sizes will be the actual input values interpreted in data coordinates.

  If auto-scaling is on, then markers will keep approximately the same screen size during zoom operations; if it's off, they will keep the same size in data coordinates.

  Marker size is also affected by the `scale` parameter.

  [Default: `false`]

`ellipseN = ellipse|crosshair_ellipse|...` (MultiPointShape)
  How ellipses are represented.

  The available options are:

  - `ellipse`
  - `crosshair_ellipse`
  - `filled_ellipse`
  - `rectangle`
  - `crosshair_rectangle`
  - `filled_rectangle`
  - `open_triangle`
  - `filled_triangle`
  - `lines`
  - `capped_lines`
  - `arrows`

  [Default: `ellipse`]

`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`]

`scaleN = <factor>` (Double)
  Affects the size of variable-sized markers like vectors and ellipses. The default value is 1, smaller or larger values multiply the visible sizes accordingly.

  [Default: `1`]

`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`]

`thickN = <int-value>` (Integer)
  Controls the line thickness used when drawing shapes. Zero, the default value, means a 1-pixel-wide line is used. Larger values make drawn lines thicker, but note changing this value will not affect all shapes, for instance filled rectangles contain no line drawings.

  [Default: `0`]

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

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

`xerrN = <num-expr>` (String)
  Error in the X coordinate.

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

`xycorrN = <num-expr>` (String)
  Correlation beteween the errors in the X and Y directions. This is a dimensionless quantity in the range -1..+1, and is equivalent to the covariance divided by the product of the X and Y error values themselves. It corresponds to the `*_corr` values supplied in the Gaia source catalogue.

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

`yN = <num-expr>` (String)
  Vertical coordinate.

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

`yerrN = <num-expr>` (String)
  Error in the Y 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