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A.4.4.3 Histogram Layer Control

The Histogram layer control (), allows you to plot a 1-dimensional histogram. It is the main control in the Histogram plot window, but is also available in the Plane plot window, so you can overplot a histogram on a scatter plot, if you can think of some reason to do that. The Histogram window starts off with one of these controls in the stack by default, but as usual you can add more by using the Add Histogram Control () button in the control panel toolbar, or the corresponding item in the Layers menu.

Each instance of this control in the stack can plot a histogram of a particular quantity from a particular table. The value to be histogrammed, and optionally an associated weighting term, are defined in the Positions tab using column names or expressions. However, the control can generate multiple layers from these coordinates; the Subsets tab controls which subsets are plotted and how each one is identified, and the Form tab provides many options for what graphics will be plotted based on the positions.

This control is a Table Data control as described in Appendix A.4.2.2. That section explains the Subsets tab; the Position and Form tabs are described in more detail below.

Position tab of Histogram layer control

Position tab of Histogram layer control

In the Position tab you enter the value of the X value, the one which is to be binned to generate the histogram. This may be a table column name or an expression. You may also optionally fill in a column name or expression in the Weight field. This weights the values as they are counted; each table row contributes a height value of the weighting coordinate to the bin into which its X coordinate falls. If Weight is not filled in, a value of 1 is assumed.

Form tab of Histogram layer control

Form tab of Histogram layer control

The Form tab lets you define how the specified data set is plotted. The Global Style and Subset Style sub-panels control shared and per-subset configration as described in Appendix A.4.2.2. The specific configuration options are:

Transparency
How transparent the bars are.
Bar Form
Type of histogram bars: filled, open, steps, spikes etc.
Thickness
Controls line thickness where applicable. This is only relevant for bar styles that draw a finite thickness line, so has no effect for Filled bars.
Dash
Controls the dash pattern of the line (solid, dots, dashes etc). This is only relevant for bar styles that draw a finite thickness line, so has no effect for Filled bars.

Histograms have some additional configuration items, as described in the Bars fixed control. When a histogram layer control is used in the Histogram window, those configuration items are found in the Bars fixed control, where they control the settings for all the histogram layers in the plot. However, if you use a histogram layer in a Plane plot, those items will appear as additional items in the Form tab and can be controlled separately for each histogram.


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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@bristol.ac.uk