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#### A.4.5.2 Size Form

The Size form () plots markers of a fixed shape whose size varies according to a supplied data coordinate. The marker size thus represents an additional dimension of the plot.

The actual size of the markers depends on the setting of the Auto Scale option. If autoscaling is off, then the basic size of each marker is the input data value in units of pixels. If autoscaling is on, then the data values are gathered for all the currently visible points, and a scaling factor is applied so that the largest ones will be a sensible size (a few tens of pixels). This basic size can be further adjusted with the Scale slider.

Currently data values of zero always correspond to marker size of zero, negative data values are not represented, and the mapping is linear. An absolute maximum size on markers is also imposed. Other options may be introduced in future.

Note the scaling to size is in terms of screen dimensions (pixels). For sizes that correspond to actual data values, the Error form may be more appropriate.

Example Size plot

Size form configuration panel

The configuration options are:

Shading Mode
  See Appendix A.4.6.
Size
  The size coordinate data values. Fill this in with a column name or expression from the table just like for the positional coordinates. The units are either pixels or arbitrary, according to the Auto Scale setting.
Shape
  Marker shape from a list of options.
Scale
  Scales the size of variable-sized markers. Adjusting the slider will make all markers larger or smaller. Alternatively you can enter a scale factor in the text field.
Auto Scale
  Determines whether the basic size of variable sized markers is automatically scaled to have a sensible size. If checked, 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 unchecked, the sizes will be the actual input values in units of pixels.

  If auto-scaling is off, then markers will keep exactly the same screen size during pan and zoom operations; if it's on, then the visible sizes will change according to what other points are currently plotted.

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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