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#### A.4.1.1 Dataset Selectors

All the graphics windows provide one or more axes on which to plot - the histogram has 1, the 2d scatter and density plots have 2, the 3d scatter plot has 3 and the spherical plot has 2 or 3. In each case you select one or more dataset to plot on these axes, and select what plotting style to use for each set. A dataset is typically a number of columns from a table (the number matching the dimensionality of the plot) and a selection of row subsets associated with that table. You select this and the plotting style(s) using the panel at the bottom of each plot window. Here is dataset selector for the 2d scatter plot:

Default dataset selector from 2d scatter plot window

The different parts of this control work as follows:

Data panel
  The Table selector gives the identifier of the table (one of the ones loaded into TOPCAT) that the data comes from.

  The Axis selectors (here X Axis and Y Axis) give the quantities to be plotted. If you click the little down arrow at the right of each selector you get a list of all the numeric columns in the chosen table, from which you can select one. If you click the little left and right arrows to the right of the selector it will cycle through all the columns in the table. However, if you prefer you can type in an expression to use here. This may be more convenient if there's a very long list of columns (another way to deal with this is to hide most of the columns using the Column Window). However, what you type in doesn't have to be a column name, it can be an algebraic expression based on columns in the table, or even a constant. In the example, the X axis is a straight column name, and the Y axis is an expression. The expression language syntax is explained in Section 7.

  The Log checkbox for each axis is used to select whether the scale should be logarithmic or linear.

  The Flip checkbox for each axis is used to select whether the axis values increase in the conventional direction (left to right, bottom to top) or its opposite.

  Some of the buttons in the toolbar shown will modify what is visible in this panel, for instance inserting new selectors to allow selection of error values. All the selectors work in the same way however.

Row Subsets panel
  This defines which row subsets for will be plotted in this window, and what plotting style should be used for them. In this case there are three defined subsets, All, galaxy and star. The checkboxes on the left indicate which ones will be displayed - here, only the latter two. Sets of points are generally plotted in the order they are selected for viewing; since points plotted afterwards can obscure ones plotted before ("underneath") this makes a difference. If you want to see a set of points without it being obscured by other ones in the plot, then deselect it and reselect it again (clicking twice in the corresponding checkbox), and this will ensure that its points are plotted on top.

  The buttons to the right of each subset name show the symbol that is used in the plot to display the data from that subset, in this case a red cross and a blue circle. These are selected automatically when the subset is first selected for viewing (the initial default style set depends mainly on how many rows there are in the selected table - many rows gives small dots, few gives big ones). However, you have a lot of freedom to configure how they appear. If you click the button with the symbol on it a dialogue will pop up which allows you to select colour, shape, transparency and so on, as well as error bar style if appropriate and things like whether fitted lines will be plotted for that subset. The options available differ according to the kind of plot, and are described along with the different graphics windows in the following subsections. The style window stays visible until you dismiss it, but if you click on another of the buttons in the Row Subsets panel its contents will change to allow you to select values for the corresponding subset. Most graphics windows have a Marker Style menu. This allows you to change all the styles in the plot at once to be members of a uniform set, for instance different coloured pixels, or black open shapes. If you select one of these it will overwrite the existing style selections, but they can be edited individually afterwards.

Dataset Tool Bar
  The toolbar shown above the data panel in the figure contains buttons which affect the dataset selector itself. The first two buttons add and remove dataset Tabs (see below) and are present for all plots. The other items configure optional selectors appearing in the Data Panel - the ones shown here are concerned with Auxiliary Axes, Point Labels and Error Bars, but not all the types of plot have exactly the same ones.
Tabs
  The example shows two tabs: Main and A; the currently visible one is A. You can select a tab by clicking on its name. In each tab you select a table and a set of columns/expressions, and if they are all filled in it will contribute points (or bars, or whatever) to the plot. The Main dataset determines the initial values for the axis labels, but the data comes equally from all of them. The numerical values of the coordinates are treated the same for all the datasets, but their meanings might be different, for instance one dataset may be plotting V magnitude against ellipticity and another plotting B magnitude against ellipticity.

  The Add Dataset () and Remove Dataset () buttons in the toolbar add a new tab or remove the selected one respectively. Initially only the Main tab is present, and this one cannot be removed.

Sometimes (high-dimensional plots, auxiliary axes, error bars) a lot of information needs to be entered into the data panel, and the bottom part of the window can get quite large. Normally, the plot in the upper part of the window shrinks to accommodate it. You can of course resize the window to gain more space, but if your screen is small you may still end up with an uncomfortably small plot. If this happens, you can use the following button from the main toolbar:

Split Window
  When this toggle button is on, the dataset selector can be resized by dragging the bar between it and the plot itself up or down. If there is insufficient space for all the components in the selector, a scrollbar will appear. When it is off (the default), the full height of the selector will be visible, and the plot will shrink to accommodate it.

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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