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#### A.4.5.9 XYCorr Form

The XYCorr () form 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         ]

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

Example XYCorr plot

XYCorr form configuration panel

The configuration options are:

Shading Mode
  See Appendix A.4.6.
X/Y Error
  The error along the horizontal and vertical directions, as column names or expressions.
XY Correlation
  Correlation beteween the errors in the horizontal and vertical 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 `x_y_corr` values supplied in the Gaia source catalogue.
Ellipse
  Ellipse graphical representation, selected from a range of options (includes also rectangles, crosses etc).
Thickness
  Controls the line thickness used when drawing polygons. 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 polygons contain no line drawings.
Scale
  Adjusts the size of variable-sized markers like vectors and ellipses. The default value is 1, smaller or larger values multiply the visible sizes accordingly.
Autoscale
  Determines whether plotted ellipses are automatically scaled to have a sensible size. If true, then the sizes of all the ellipse 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.

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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk