Desktop Survival Guide
by Graham Williams


Image ggobi-control-xyplot-age-education
When you startup GGobi (Execute the GGobi option) two windows will appear: one to control the visualisations and the other to display the default visualisation (a two variable scatterplot). The control window is as displayed to the right. It is a basic window with menus that provide the overall control of the visualisations. Below the menu bar you will see XY Plot which tells us that we are displaying a two variable scatterplot. On the right hand side is a list of the variables from your dataset, togther with buttons to choose which variable to plot as the X and the Y. By default, the first (Age) and second (Employment) are chosen. You can choose any of your variables to be the X or the Y by clicking the appropriate button. This will change what is displayed in the plot.

From the Display menu you can choose a New Scatterplot Display so that you can have two (or more) plots displayed at a time. At any one time just one plot is the current plot (as indicated in the title) and you can make a plot current by clicking in it.

Image ggobi-audit-xyplot-age-employment       Image ggobi-audit-xyplot-age-income

Image ggobi-audit-autobrush-adjusted
For our purposes we are usually most interested in the relationship between the values of the variables for entities that have an Adjusted value of 1 or 0. We can have these highlighted in different colours very easily. From the Tools menu choose Automatic Brushing. From the variables list at the top of the resulting popup window choose Adjusted. Now click on the Apply button and you will see that the 1,537 points that have a value of 0 for Adjusted remain purple, whilst those 463 entities that have a value of 1 are now yellow. This will apply to all displayed plots.

Image ggobi-audit-xyplot-age-employment-adjusted       Image ggobi-audit-xyplot-age-income-adjusted

Image ggobi-audit-control-scatter-matrix-default
The Display menu provides a number of other options. The Scatterplot Matrix, for example, can be used to display a matrix of scatterplots across many variables at the one time. By default, the first four variables are displayed, as illustrated here, but we can add and remove variables by selecting the appropriate buttons in the control window (which is now displaying only the choice of X variables. You can also use the Automatic Brushing that we illustrated above to highlight the adjusted cases. Such matrix scatterplots are effective in providing an overview of the distributions of our data.

Image ggobi-audit-scatter-matrix       Image ggobi-audit-scatter-matrix-adjusted


Image ggobi-audit-parcoord-example


Image ggobi-audit-barchart-age

The thing to note here is the two arrows down the bottom left side of the plot. Drag these around to get different width bars.

A scatterplot over very many points will sometimes be solid black and shows little useful information. In these cases a bagplot may be useful.

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