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#### A.4.4.4 Matrix Layer Control

The Matrix layer control () is available only for the Corner Plot. The Corner Plot window starts with one of these in the stack by default, and you can add a new instance by using the Add Position Control () button in the control panel toolbar, or the corresponding item in the Layers menu.

This is the control that supplies all the plots for the Corner plot. Each instance of this control in the stack does plotting for a set of multidimensional points from a single table. The set of points is defined in the Position tab as a column name or expression for each plot coordinate, while the Fill tab provides some controls that can help to fill in in the Position tab. The control can generate multiple different layers from these positions; the Form tab provides many options for what graphics will be plotted based on the positions, and the Subsets tab controls which subsets are plotted and how each one is identified.

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

Position Tab

Position Tab of Matrix layer control

To select the quantities that will be plotted against each other, in the Position tab select column names or enter algebraic expressions into the fields marked X1, X2, X3 etc. Grid cells will be plotted for each distinct pair (scatter-plot-like) and single (histogram-like) combination of the specified coordinates.

This position entry panel has a few features not found in the other plot types:

- The Add () and Remove () buttons can be used to increase/decrease the number of fields visible for coordinate entry.
- There is a Grab Handle () by each coordinate; you can drag this up or down using the mouse to re-order the coordinate values you have entered.
- There is a small Delete button () by each coordinate; you can use this to remove the adjacent value from the plot (all the ones below will move up one).

Fill Tab

Fill Tab of Matrix layer control

The Fill tab lets you choose columns from the table you are plotting, and use them to populate the selectors in the Position tab. Moving and selecting the columns in this tab does nothing on its own, but if you hit one of the buttons under the Populate Position tab heading on the right, the Position tab, and hence the plot, is updated accordingly. This doesn't do anything that can't be done by filling in fields in the Position tab directly, but it can be a quicker and more convenient way to do it.

The left hand panel shows all the columns in the input table (as selected in the Position tab). They can be reordered using the mouse by dragging them with their Grab Handle () and the checkbox by each one can be selected. The three buttons at the top of the right hand panel also affect this list:

- : Reloads the column list from the table; this ensures that the order of the columns in the list is the same as the current column order in the table it comes from
- : Selects all the columns in the list
- : Unselects all the columns in the list

When you have selected the columns you're interested in, you can hit one of the buttons below the Populate Position Tab heading.

Selected
  All the columns selected in this window will be used to fill in the coordinate selectors in the Position tab.
Pair Differences
  All pair differences between the columns selected in this window will be used to fill in the coordinate selectors in the Position tab. So for instance if the four columns `A`, `B`, `C` and `D` are selected in the list here, then clicking this button will fill in the selectors in the Position tab with X1=`A-B`, X2=`A-C`, X3=`A-D`, X4=`B-C`, X5=`B-D`, X6=`C-D`. The number of replacement coordinates (hence the linear size of the grid) will be N(N-1)/2 so it can end up generating a lot of plots. This is a rather special-interest option, but it can be handy for instance when working with sets of correlated parameters such as source magnitudes.
Pair Ratios
  Like the Pair Differences above, but instead of differences, ratios are used, so the selectors would be filled in with X1=`A/B`, X2=`A/C`, X3=`A/D`, X4=`B/C`, X5=`B/D`, X6=`C/D`. This could be useful when working with sets of fluxes.

Note these buttons will only be enabled if they would result in a reasonable number of position coordinates according to the current column selection. If any of them would result in fewer than two, or more than some reasonable maximum (64 at time of writing), it is disabled.
Form Tab

Form Tab of Matrix layer control

The Form tab lets you define how the specified data set is plotted. The stack on the left gives a list of forms currently being plotted, and the panel on the right shows the detailed configuration for the currently selected form.

When first added, the stack contains two entries, Mark and Histogram. This gives you a scatter plot on all the off-diagonal grid positions, and a histogram on all the diagonal ones. However, there are a number of other forms that you can plot as well or instead of the simple markers; two-coordinate plots like contours and linear fits on the off-diagonal positions, and one-coordinate plots like KDEs on the diagonal positions. You add a new form to the stack by clicking on the Forms button, which gives you a menu of all the available forms. You can remove a form by selecting it and clicking the Remove () button in the same menu. You can also activate/deactivate the entries in the stack with the checkbox and move them up and down with the drag handle as usual.

The detail panel of each form depends on the form itself. It is divided into the following panels, though not all forms have all the panels.

Shading
  The shading mode controls how points are shaded based on their chosen colour. The various options are described in Appendix A.4.6. Depending on the mode there may be more settings to fill in here.
Coordinates
  If additional coordinates are required for this form, for instance the size of error bars, you need to enter the column or expression here. Each coordinate effectively adds another dimension to the plot. Many forms, like Mark, do not require any additional coordinates.
Global/Subset Styles
  Controls the style details for the chosen form; see Appendix A.4.2.2.

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Next: Healpix Layer Control
Up: Layer Controls
Previous: Quad Position Layer Control

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