A.3.6.1 Defining Subsets From Plots

When columns are plotted against each other in the Plot Window, it becomes easy to see groupings of the data which may not be otherwise apparent; a cluster of (X,Y) points representing a group of rows may correspond to a physically important grouping of objects which you would like to treat separately elsewhere in the program, for instance by calculating statistics on just these rows, writing them out to a new table, or plotting them in a different colour on graphs with different coordinates. This is easily accomplished by creating a new Row Subset containing the grouped points, and the Plot Window gives you two ways to do this.

The simplest way is to zoom the plot so that only the points you want to identify are visible (by dragging the mouse down-and-right to zoom in or up-and-left to zoom out) and hitting the New Subset From Visible () toolbar button. This defines a subset consisting of all the points that are currently visible. This has the limitation that only a rectangular grouping of points can be selected.

A much more flexible way is to draw a region or regions on the plot which identify the points you are interested in. To do this, hit the Draw Subset Region () toolbar button. Having done this, you can drag the mouse around on the plot (keep the left mouse button down while you move) to encircle the points that you're interested in. As you do so, a translucent grey blob will be left behind - anything inside the blob will end up in the subset. You can draw one or many blobs, which may be overlapping or not. If you make a mistake while drawing a sequence of blobs, you can click the right mouse button, and the most recently added blob will disappear. When you're in this region-drawing mode, you can't zoom or resize the window or change the characteristics of the plot, and the Draw Subset Region button appears with a tick over it () to remind you you're in it. Here's what the plot looks like while you're drawing:

Region-Drawing Mode

Region-Drawing Mode

When you're happy with the region you've defined, click the toolbar button again.

In either case, when you have indicated that you want to define a new row subset, a dialogue box will pop up to ask you its name. As described in Section 2.1.1, it's a good idea to use a name which you haven't used before, and which is just composed of letters, numbers and underscores. When you enter a name and hit the OK button, the new subset will be created and the points in it will be shown straight away on the plot using a new symbol. As usual, you can toggle whether the points in this subset are displayed using the Row Subsets box at the bottom of the Plot Window.